Well I've been looking at market place and found 1" tempered laminate. Would that work it's exactly 6ft×3ft. They sent me pictures and it looks crystal clear and I can get it for $50. I also found a big sheet of bullet proof glass which I'm assuming since it stops bullets it should hold water cost a little more but still $200 for a big sheet seems reasonable
Sounds like youre talking about insulated window panels which would not be a good idea. A 1" laminated window panel is two layers of thinner glass with an insulating air layer between them. To be fair, ive never seen anybody use this on a tank but i dont think it would work. The thin glass panels, probably 3/16", are nowhere near strong enough for the depth youre talking.
And no i would not recommend the "bulletproof" glass either. 36" of water depth/pressure is much more force than a typical bullet will carry. I dont know what exact type of polymer youre referring to or the thickness so i cant really say for sure if it would work.
You should familarize yourself with plate glass and cell cast acrylic---the only two materials recommended for building aquariums. I dont mean to be coarse but youre way out in left field talking about 4" thick glass, bulletproof glass, etc...it sounds like youre on a budget which i get but i would do a lot more research unless you want to waste a lot of time and effort on this with repurposed, ineffective materials.