well if you're using 1/4" glass for the bottom, I'd think that resting the edges of the walls on it, and subjecting it to water pressure may result in the bottom cracking. you could seal the bottom to the inside of the sides, instead of the bottom edges of the sides, and even put external support (by way of a molding or something around the outside) so the bottoms don't want to push outward too badly.
Personally I'd be tempted to build the base out of whatever (wood, concrete, etc) with a rabbet all the way around for the glass sides to sit in. Seal the bottom with epoxy or polyester, silicone in the sides, and bob's your uncle. No flimsy 1/4x4'x8' glass sheets.