I've done something like that a number of times.
(Well not the "blind test".)
Take a really great 5' 10" tri-fin and added 2 extra plug sets on the sides of the center fin.
Maybe about half way to the rail.
Or vice-versa.
The difference in ride is that the 4 fin setup has more bite, projection and will bank a turn more on edge, with consistent drive through the turn, since there's be a rear fin always engaged.
(Too much fin can become too tracky though.
The key is to find a comfotable feeling fin size and template.)
Where, as a tri-fin, the same board needs to be turned with a bit more care, kinda turned in little stages to manage keeping the center fin engaged.
Edited:
I forgot (sick child calling
) to add that the tri-fin set up was easier to turn in general.Tail slides are a breeze. They definately pivot better at a tip of the hat.
Though, while in the barrel the center fin disengages at times and you have to let the board just ride it out relying on the side fins and rail for hold. Can be exciting!
In tri-fin mode, if I drove the turn as hard, and banked over, as I would in 4 fin mode it'd over steer and or spin out.
One could approximate the tri-fin turn feel by cutting down the size of the dual back fins. It'll still feel different enough to notice.
You can spin it out as well, but the release is more gradual.
Note:
I set the rear fins of my 4 fins with little angle at all.
They are also symetrically foiled.
I have also used asymatric back fins and they are a little looser still.
I have also set the back fins, on many a board, with an untold variety of angles.
You could get even closer to the tri-fin feel, using more acute angles on the back fins.
I still think you'd notice the difference.
The tri-fin feel is quite distinct.
It's all great fun regardless.
