(adds late game, details)
* Man City recover from two goals down to beat Watford 4-2
* Aguero treble and Kolarov strike put City in fifth round
* Liverpool, Everton and holders Wigan all reach last 16
* Sunderland beat last non-league survivors Kidderminster
By Martyn Herman
LONDON, Jan 25 (Reuters) - Second-tier Watford were half an hour away from a seismic FA Cup shock at Manchester City on Saturday before Sergio Aguero ruined the script with a hat-trick as his side stormed back to reach the last 16 with a 4-2 win.
Only European champions Bayern Munich had returned victorious from City's Etihad Stadium fortress this season but goals by Fernando Forestieri and Troy Deeney had the Watford fans dreaming of a famous result.
The London side were still leading 2-0 after an hour but eventually succumbed to the siege as Aguero cancelled out the deficit, Aleksandar Kolarov put City ahead three minutes from time and their Argentina striker completed a treble to maintain Manuel Pellegrini's side's four-pronged assault on silverware.
"I wanted to change all of my players at half-time, all 11 of them," a relieved Pellegrini said.
"The whole team was a disaster in the first half."
Liverpool had no such problems as they reached the last-16 with a 2-0 win at second tier Bournemouth - Victor Moses and Daniel Sturridge scoring for the seven-times winners who were made to work hard on the south coast.
Moses opened the scoring after 26 minutes and Sturridge linked with Luis Suarez on the hour to ease Liverpool through and make sure there was no repeat of last season's humbling defeat to third tier Oldham Athletic.
Kidderminster, the only survivors from outside England's top four divisions, battled valiantly at League Cup finalists Sunderland but slipped out 1-0 while fourth tier Southend United were beaten 2-0 at home by Premier League Hull City.
League One Sheffield United, who face Premier League Fulham on Sunday, are now the only side not in the top two divisions still in the competition after Everton crushed Stevenage 4-0 in Saturday's late kickoff.
Everton paid a price for their victory though with Costa Rica fullback Bryan Oviedo suffering a suspected broken leg.
Holders Wigan Athletic, who were relegated to the Championship after beating Manchester City in last year's final, kept alive their hopes of an unlikely successful defence of the trophy with a 2-1 win over top flight Crystal Palace.
Apart from Palace's exit there were no other surprises with Wilfried Bony's double helping Swansea City triumph 2-1 at Birmingham City, Cardiff City winning 1-0 at Bolton Wanderers and Southampton easing past second-tier side Yeovil Town 2-0.
Championship sides Brighton and Hove Albion and Charlton Athletic are also in Sunday's fifth round draw, as are three-times winners Sheffield Wednesday who ended the hopes of fourth-tier Rochdale, winning 2-1 away despite finishing with 10 men.
Chelsea host Stoke City on Sunday, while Premier League leaders Arsenal progressed on Friday with a 4-0 victory over Coventry City at The Emirates.
Even diehard Watford fans would have feared the worst as their side ventured north to Manchester to face a City side who have trounced the likes of Arsenal, Tottenham Hotspur and Manchester United at home this season.
City began the match with 106 goals already in the tank from the four competitions they are contesting, the majority of them at home, and a strong lineup.
But they fell behind after 21 minutes when Forestieri fired through the legs of goalkeeper Costel Pantilimon.
Nine minutes later the side that are 15th in the Championship extended their lead through Deeney to send their 3,500 travelling fans into delirium.
City boasted an attacking quartet of Stevan Jovetic, Edin Dzeko, Aguero and Jesus Navas after the break and once Aguero clinically pulled a goal back on the hour there was a sense of inevitability about the outcome.
Aguero curled home the equaliser in the 79th and, after Kolarov's weak shot somehow got past Watford keeper Jonathan Bond three minutes from time, Aguero completed the turnaround with his head to maintain City's four-goal average at home. (Reporting by Martyn Herman, editing by Pritha Sarkar and Josh Reich)