Watford can head home from the Etihad Stadium with their heads more than held high after coming within less than ten minutes of forcing an FA Cup fourth round replay, only for Manchester City’s multi-million-pound assembled array of stars to finally prevail 4-2.

Only the most optimistic fan would have given Watford a hope before kick-off but by the interval a huge shock was very much on the cards.

The Hornets arguably already deserved to be in front before the 20th minute when Troy Deeney slipped in Fernando Forestieri and he kept his calm to beat Costel Pantilimon.

And that lead was doubled nine minutes later when more shocking City defending allowed Ikechi Anya to play in Deeney and like his teammate before, he also showed admirable composure to double the Hornets’ advantage.

Sergio Aguero halved the deficit just before the hour-mark and then the hosts inevitably cranked up the pressure. The Hornets were able to hold out until 12 minutes from the end when Aguero curled home a superb equaliser.

A replay was still on the cards though, but the manner in which City took the lead with four minutes of normal time remaining was horrible for Jonathan Bond as the Hornets keeper let an Aleksandar Kolarov shot squirm through his grasp.

There was still time for City to add a fourth – their 110th goal in all competitions – as goal-machine Aguero completed his hat-trick but the victors will know they had to dig deep as their opponents, for much of the match, were on course to cause a sensation and become only the second time alongside European champions Bayern Munich at the Etihad this season.

Beppe Sannino made five changes to the team which extended Watford’s unbeaten away run to an eighth game by holding out for a 1-1 draw at Bournemouth last week despite being reduced to ten men.

More than half the alterations were enforced – with Fitz Hall suspended and Iriney and Lewis McGugan both injured – while Manuel Almunia and Diego Fabbrini dropped to the bench.

Their places were taken by, in what appeared to a new-look 4-5-1 formation, Bond, Lloyd Doyley, Anya, Sean Murray and Forestieri.

New signing Mathias Ranegie was on the bench along with the fit-again Marco Cassetti and, for the first time this season, Tommie Hoban.

Manuel Pellegrini shuffled his pack once again and made six changes to the side which easily saw off West Ham United 3-0 in midweek to complete a 9-0 aggregate win and emphatically book a Capital One Cup final trip to Wembley to face Sunderland.

Into the side came Micah Richards, Martin Demichelis, Kolarov, Jack Rodwell, skipper Yaya Toure and Edin Dzeko. Alvaro Negrado missed out through injury, Matija Nastastic, Gael Clichy and Fernandinho dropped to the bench and Dedryck Boyata and Javi Garcia were not involved.

The Hornets in fact started in a 3-4-2-1 formation and had an effort, of sorts, inside the opening 30 seconds when Troy Deeney fired well wide from an impossible angle towards the right edge of the penalty area.

Watford had a far more promising two minutes later when they were awarded a free-kick on the edge of the area after Toure bundled over Forestieri. However, the Argentine’s set-piece went straight into the wall and then Cristian Battocchio struck a half-volley over from the edge of the 18-yard box after the rebound sat up nicely for him.

The visitors continued their positive start and had a very good case for a penalty in the sixth minute when Davide Faraoni clipped a ball over the top, Anya outpaced Demichelis, lifted the ball beyond Pantilimon and was then clattered by the keeper. But Sannino was left smiling ruefully on the sidelines as referee Kevin Friend briefly allowed play to continue before the ball was knocked into touch so Anya could receive treatment.

City had barely got started and their collective mood was not improved in the tenth minute when Dzeko was booked after squaring up to Gabriele Angella in the penalty area as the hosts prepared to take a free-kick.

Pellegrini’s men though, did have their first opening soon after when the Hornets switched off at a corner, allowing Kolarov to get totally free at the near post and flick a Jesus Navas set piece across the danger area but the defence manage to scramble the ball behind.

But it was Watford’s chance to create an even better opening in the 16th minute when, after a Toure shot had been blocked, they broke in numbers with Forestieri carrying the ball around 60 yards. However, his final through ball was to have too much pace on it as he tried to put slide in Faraoni, who was in the clear in the penalty area.

Sannino’s men should have done better with that opportunity but in the 20th minute, 3,352 fans at one end of the group were roaring in collective jubilation as the Hornets made the breakthrough – and not against the run of play.

Faraoni played the ball to Deeney, who clipped a pass round the corner and Forestieri timed his run superbly to get clear of the static City rearguard and he kept his composure superbly to beat Rodwell and make it 1-0.

Bond comfortably held a 20-yard effort from Rodwell as the home side sought an instant response but then the keeper almost got in a muddle with Angella as Navas lurked before the Italian defender frantically cleared the ball behind.

But the highly unlikely assumed borderline unthinkable proportions in the 29th minute when the Hornets sensationally doubled their lead.

City were frankly a shambles at the back as Watford got possession on the left, built steadily before Anya slipped in Deeney who, like Forestieri before him, kept his head and comfortably beat Pantilimon to reduce the vast majority inside the Etihad to stunned silence.

City tried to regroup after going two down, but didn’t really threaten as their opponents remained resolute, keeping their shape and organisation superbly as the clock started to run down towards half-time.

The hosts tried to build some momentum in the final five minutes of the half but Doyley did superbly to dispossess Aguero as he tried to get a shot away and then a superb block thwarted a Kolarov drive.

But City were still looking vulnerable on the counter and three minutes before the interval the Hornets had another promising situation when the ball was worked to Forestieri on the edge of the area. His shot was blocked and then Faraoni sliced the angled rebound wide.

Soon after though, a low Anya cross from the right was allowed to run all the way across the area and Daniel Pudil did well to get on the end of it but he had to take the shot first time and hit the side netting.

But 2-0 it stayed at the break and it was clear a defensive reshuffle was on the cards for City in the second half as Vincent Kompany and Pablo Zabaleta were both left on the pitch during the interval to warm up.

The changes were duly made as Richards and Rodwell were hauled off by Pellegrini at the start of the second period as the hosts immediately started on the front foot.

Pudil was booked three minutes after the restart for a foul on Navas and then the Hornets were fortunate to escape when they were sloppy at a corner, but two runners towards the near post could not get a clean head on Kolarov’s delivery from the left.

Watford got through the first ten minutes and had a couple of ambitious efforts of their own, with Anya and Pudil both trying half-volleys from the edge of the penalty area after corners were partially cleared.

City had their first clear opening of the afternoon when Dzeko found space in the area and played a one-two with Navas before doing little more than roll a shot through to Bond from barely eight yards out.

Pellegrini played his final card in the 57th minute as Stevan Jovetic replaced Marcos Lopes and the substitute was lurking behind Dzeko as he failed to get his head on a delivery from the right when superbly placed in front of goal.

But City were back in the contest two minutes later when Bond did very well to deny Dzeko but the keeper was helpless as the ball broke for Aguero who slammed home his 23rd of the season.

Sannino instantly replaced Forestieri with Cassetti and then Dzeko continued to add to his list of poor finishes as tamely side-footed a shot through to Bond after Kompany had threatened to shoot after reaching the edge of the penalty area.

But 2-1 it remained as the clock ticked past the 70-minute mark as Angella produced a terrific block to repel a Toure shot. An afternoon of industry and no little quality at times from Murray was ended four minutes later when he was replaced by Almen Abdi.

The Hornets though, were to be undone for a second time in the 78th minute when City’s undisputed quality came to the fore.

Navas picked up the ball in his own half and ran across the pitch from right to left but forward as he drifted past Faraoni before finding Dzeko. He slipped the ball to Aguero on the right side of the area and the Argentine bided his time before curling a superb left-footed finish past the helpless Bond and into the top corner to make it 2-2.

Fabbrini replaced Faraoni with six minutes remaining as Watford tried to hold on for the replay but it was not to be as a horrible moment for Bond saw City score their third with four minutes of normal time remaining.

Kolarov connected well enough with a daisy-cutter from 25 yards but the Hornets keeper would have expected to deal with it, only for the ball to squirm through his grasp and between his legs as he bent down to claim.

Anya did shoot wide at the other end but having got in front, there was no great surprise that City were able to maintain their four-goals-a-home game average in injury time when as the Hornets tormentor in chief in the penalty area completed his hat-trick.

Bond started to look as if we could come to deal with a deep cross from the right but hesitated and Aguero didn’t need a second invitation to complete his treble after the break and book City’s place in the fifth round as they continue their trophy pursuit of four fronts.

Manchester City: Pantilimon; Richards (Zabaleta 46), Demichelis, Lescott, Kolarov; Navas, Rodwell (Kompany 46), Toure, Lopes (Jovetic 57); Aguero, Dzeko. Not used: Hart, Nastasic, Clichy, Fernandinho.

Watford: Bond; Faraoni (Fabbrini 84), Doyley, Ekstrand, Angella, Pudil; Anya, Battocchio, Murray (Abdi 74), Forestieri (Cassetti 60); Deeney. Not used: Almunia, Ranegie, Hoban, O’Nien.

Bookings: Dzeko for off-the-ball clash with Angella (10); Pudil for a foul on Navas (48).

Attendance: 46,514 (3,352 away).

Referee: Kevin Friend.