Following their midweek win over Blackburn Rovers, Anthony McNamee stung the hapless Hammers with a 42nd-minute winner for the Hornets that gives them a place in Monday's fifth round draw and a welcome distraction from the relegation battle that has haunted their campaign.
These two struggling sides had shared a 1-1 draw at Vicarage Road in their second Premiership match of the season filled with hope but, five months on, both teams are now clinging onto top-flight status for dear life.
And while both managers would have come into this tie willingly swapping priceless Premiership safety for an FA Cup run, Aidy Boothroyd admits that the rock-bottom Hornets - who eliminated Stockport County in the last round - will take the shot in the arm of any win in any competition these days.
Watford made two changes from the side that beat Blackburn as Jay DeMerit and McNamee came in for Danny Shittu and Will Hoskins but, having just notched only their second league victory of the season, they soon found last season's beaten finalists putting them under pressure in the opening stages as Bobby Zamora sent a looping header bouncing onto Ben Foster's crossbar as early as the second minute.
The Hammers' striker was one of four fresh faces from the side that had controversially drawn 2-2 draw at Newcastle United, as new signing Lucas Neill - reportedly on £70,000 per week - made his debut, while Jonathan Spector, Shaun Newton and Zamora each came into the starting line-up.
And Foster then had his fingers fried by Nigel Quashie and Shaun Newton, who each unleashed scorching shots before Jordan Stewart bundled Christian Dailly's header off the line midway through the first half.
Having weathered the early East End storm, Watford - who had seen an earlier Damien Francis tap-in rightfully ruled out by an offside flag - gradually grew in confidence as they counter-attacked with both pace and power.
Indeed, on 35 minutes, they looked all set to take the lead, only for Carlton Cole to nod Darius Henderson's goalbound header out from underneath the crossbar at the last moment.
But Boothroyd did not have to wait too long for his side to get their noses ahead and, three minutes before the break, Henderson got in front of both Dailly and the shockingly stranded Carroll to flick Hameur Bouazza's cross into the path of McNamee, who cleverly sent a 15-yard overhead kick into the unguarded net.
The frustration at seeing his side's grip on the game slacken was just too much for debutant Neill, who was booked for an unsightly scythe on Bouazza on the stroke of half-time and, early in the second half, to visiting chants of 'you're only here for the money' a knee injury forced him to limp off.
As West Ham tried to get back on terms, Zamora fired at Foster before heading over, while the breaking Cole lost control at the vital moment and found himself duly booked for a desperate dive.
There was simply no sign of the West Ham side that had so clinically dispatched Brighton in the last round and, with the exit lights flashing, Curbishley introduced Teddy Sheringham at the expense of Newton.
But, even with a three-pronged attack - apart from Spector's late attempt - the dejected Hammers did little to prevent themselves from trudging off to chants of 'you're not fit to wear the shirt'.