[Griznatch]

Cinder blocks and 2x4s work pretty good - probably aren't going to get 4 shelves though. Like was mentioned before, you need decent space above the tanks on lower shelves, or maintenance will be a nightmare. Check Aquarium co-ops video on 2x4 shelving for tanks.

Industrial shelving would work, like in a warehouse but it's pretty expensive.

Just a thought, but how are you going to do anything on a 4th shelf? With even small tanks that's going to be pretty far up...