Freddy Eastwood and Daryl McMahon both hit stunning double strikes to maintain a 100% record for the under-19s this season, while Kevin Keen's youngsters showed fantastic character to battle back from a goal down with efforts from Darren Blewitt and Sos Yao.
Tony Carr's team opened the scoring in the 23rd minute when Eastwood collected the ball midway inside his own half, turned and ran towards goal before unleashing a terrific left foot shot from 25 yards out that flew in the top corner.
The Hammers then had goalkeeper Glenn Jackson to thank for holding their lead as he pulled off two excellent saves before the break to deny the hosts.
On 60 minutes, Eastwood doubled his tally and his team's lead when he spun magnificently in the penalty area to rifle an unstoppable shot into the net after Watford had failed to clear a free-kick.
With 10 minutes remaining, McMahon got in on the act with a virtual carbon copy of Eastwood's opener, running at the Hornets defence before finding the top corner with a right-foot shot from 25 yards out.
The Irish midfielder then rounded off an excellent performance with a direct free-kick from 20 yards out that flew into the bottom corner.
Said Tony Carr afterwards: "It was a fantastic result, although to be honest the game was a lot more even than the scoreline suggests.
"Watford played very well and had a lot of possession, but it just seemed to be one of those days when everything went right for us.
"We scored four absolutely tremendous goals and played some great football. It has been a very good start to the season, winning all of our first three games and the players have a lot of confidence."
Team: Jackson, McClenahan, Tattam, Ferdinand, Ward, Lumsden, Allen (Akinsete 55), Mehmet (Pearson 74), Eastwood (Morris 85), Cole, McMahon. Unused: Fletcher.
Meanwhile, Kevin Keen was left delighted after his under-17s grabbed a deserved first win of the season against the Hornets at Little Heath.
After starting the better of the two sides, Hammers were dealt a blow when Watford took the lead against the run of play after 20 minutes.
However, the hosts fought back to show excellent character and dominated proceedings for the remainder of the game.
The equaliser arrived on 67 minutes, when defender Darren Blewitt headed in Danny Bunce's free-kick at the far post.
With 15 minutes left, Keen sent on substitute Sos Yao, and the schoolboy striker had a winning impact.
With virtually his first touch of the ball, Yao wriggled into the penalty area and struck a fine shot that went in off the far post to give the Hammers a lead that they held on to for the final eight minutes.
Said Kevin: "I'm very, very pleased with the boys, it was an excellent performance and we thoroughly deserved the victory.
The players have been a little bit anxious after losing their first two matches but I have said all along that their attitude has been fantastic and they have got their reward today.
"It's nice to get the first win out of the way and hopefully the players will settle down a bit more now and progress from here.
Team: Behcet, Wright (Henry 82), Laws, Parrington, Blewitt, Cohen, Tucker, Noble, Bunce, Lee (Yao 75), Collington (Stokes 80). Unused: Read, Singh.