Hitchin finished the league part of the rugby season with a fine home win but Letchworth were denied a third-placed finish in the final moments.
The victory for the Hedgehogs came over Chiswick and by 29-14 to leave them seventh in Counties One Herts Middlesex.
Hitchin played into the strong wind in the first half and for 20 minutes there was little to choose between the sides.
Chiswick were the first to blink, collecting a fumbled Hitchin pass and running in from close to halfway under the posts.
But by the break it was the home side who were in front.
Some good passing between forwards and back down the left allowed Simon Taylor to go under the posts a few phases later, Pete Stanbury adding the extras and then Ewan Rawlings finished off after a break by Jack Maydom to make it 12-7 by the end of the half.
With the wind in the second period though, Hitchin were on top almost throughout.
They got try number three when a penalty was kicked to the corner, the resulting maul stopped but popped out to Frazer McGown to cross unopposed.
The bonus point try came from another Hitchin line-out, this one on the Chiswick 22.
It came out on the visitors' side but James Bolter pounced on it and evaded a couple of defenders to score, Pete Stanbury getting his second conversion.
The final try of the day belonged to the returning Will Evans who weaved his way in from full-back.
Chiswick did get a converted consolation try on the last play but it was way too little and too late.
Letchworth meanwhile knew a win at home to bottom of the table Westcliff at Legends Lane would see them claim third place in Regional One South East.
And they were all set to do so until a penalty try in the final moments left the sides tied at 38-38, with Harpenden's win against Sudbury taking them above their Hertfordshire neighbours.
Fourth was still a respectable finish for Letchworth who were only promoted back to the level last season and it does give them something to build on for the next campaign.
Here it was Letchworth who had the advantage in the first half but without truly making it pay.
Luke Mongston got the initial score, converted by Josh Sharp, but by the time Tom Lewis drove over, Westcliff had two converted scores of their own to leave the sides level at 14-14.
Sam Valpe got the first try of the second half, finishing off a fine backs move, but Westcliff moved into a 31-19 lead with a little more than 20 minutes to go.
Tries from Charlie Parkhouse and Sharp, who converted the first but not his own, made it 31-31 and Letchworth looked to have won it when Parkhouse went over for his second, Sharp again adding the kick.
But the final twist saw Westclilff given seven points when a no-arms tackle bundled their winger into touch.
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