Head coach Rhodri Thomas praised his side as they inflicted just a second league defeat of the season on champions GHK at Neilson Park on Saturday.
A depleted Reds squad, missing a number of regulars from the pack, produced a superb performance to stun the Glasgow side, already confirmed as champions, in their final league game.
Veteran duo Alan Laidlaw and Mark Morrison returned alongside another experienced campaigner Graeme Dykes in the forwards, while the evergreen skipper Colin Stuart moved from hooker to number eight.
The backline was more settled, while there was a possible 1st XV debut from the bench for Ally Stewart, having impressed for the 2nds this season.
Thomas' side made the worst possible start to the match as they fell behind early on, with Danny Campbell going over for the visitors' first try.
When Campbell crossed for his second try of the half, the Reds found themselves 0-12 down and up against it.
Thomas' side got back into the match thanks to a moment of individual brilliance from full-back James Ferguson, who broke the line superbly, and backed himself against the covering defenders to score a 60-metre solo try.
Gareth Pritchard converted to bring the hosts within seven points, but they were dealt another blow when hooker Kyle Munro was sent to the sin-bin.
The already-crowned champions took full advantage to score their third try, as Jez Hanks went over.
A successful conversion meant Haddington were 7-19 behind at the interval.
The Reds, though, fought hard up front, with Morrison, Dykes and Harry Hamilton to the fore as they got back into the match.
A second try reduced the deficit further, with Alan Laidlaw the man scoring after a well-worked rolling maul.
Pritchard, goaled the extras to make it 14-19.
And the comeback was completed when they scored a third try to draw level.
With the visitors' Nick Hurn in the sin-bin, the Reds produced an excellent team move which put Kyle Adams away to score.
Pritchard missed with the conversion attempt, but landed a penalty to put the home side ahead for the first time at 22-19.
Adams showed his acrobatic skills to beat the defence, and avoid the corner flag, to score his second of the afternoon in the corner (27-19).
That wrapped up a bonus point, and although GHK crossed for their fourth late on, the Reds held on for a stunning comeback win.
And head coach Thomas was delighted with the result.
He said: "We had set ourselves a target of scoring four tries and we achieved this, and secured fourth spot in the league.
"Finishing the season in this way is a real boost for the team and gives us a great platform to start from next season.
"GHK are worthy champions, so to get a bonus point win against them was hugely positive."
Haddington: James Ferguson, Kyle Adams, Kyle Hamilton, Liam Clutton, Mark Robertson, Gareth Pritchard, Jack Hamilton; Alan Laidlaw, Kyle Munro, Phill Smith, Harry Hamilton, Jonny Riddell, Graeme Dykes, Mark Morrison, Colin Stuart (c)
Replacements: Connor Munro (for Morrison) Andrew Durkin (for Robertson), Ally Stewart (unused)