Icarus, 4mm is a bit of a big gap, but on a trainer, it may be fine. It is possible to seal the gap with covering, but that process is probably no easier than cutting and re-hinging.

I would probably give it a try as-is. At low speeds, it will probably work just fine.

Where do you plan on flying? I fly at the 495th field in Tewksbury.

Good luck