Just some information, The vertical strength of a 2x4 @ 8' is 1 tonne. So @ 2' the strength is ~ 4 tonnes x 4 legs is 16 tonnes. This is extreme tho, and any twists/defects would reduce the efficiency of the wood. It would also have to be sturdy. If the stand has a proper face/back and sides to keep everything square (either cross bracing, or a 1/2" plywood frame), a 3' high 2x4 frame can hold up even a 455Gal tank (> 2 tonnes) tho I would err on the safe side and use more then 4 legs, probably 2x8, since the horizontal flex point of a 2x4 is quite low by comparison, even on edge.
120 Gal SW, 150lbs Prem. Fiji, 23Gal Sump, 2x250w 15K MH, 4x65w Actinic
3 x Halichoeres Chrysus (1 surfed), 3 x Nemateleotris magnifica, 1 x Centropyge bicolor, 11 x Scarlet Hermits, 6 x Zebra Hermits, 40 x Astrea Snails, 6 x Nassarius Snails, 3 x Cerith Snails