instead of adding them here, ill just give you the link cause its easier haha.
here you go http://www.monsterfishkeepers.com/forums/showthread.php?t=87045
its overbuilt for sure, and has been set up since then, should hold alot more than that...personally i think staining it looks alot better then my crap paint job, might skin it at another date bc of that.
4x2s will support a tremendous amount of weigth in end on compression. Think about it, they hold up houses all the time.
What you need to do is brace them so they cant be pushed over and the whole stand fold up - that would be pretty disasterous.
The easiest way to do this is to clad the 4x2 structure with the plywood as suggested. Use the plywood both as a nice cladding to the stand AND a structural bracing component.
A 4x2 box clad in ply will support almost anything !!
The 80 gallon stand has a piece of 5/8" plywood across the back for sheer strength. I like 3/8" galvanized lag-bolts and thru-bolts for that overkill effect. That stand would hold up an F-250 if properly balanced!
The 135 stand has been "skinned" and I enclosed the side and top behind the tank. I used 3/4" plywood and 3/4" quarter-round molding. To finish the corner miter the pieces at a 45 and then use a rasp and a sanding block to round it off.
I hope this helps or at least turns on a light bulb in your noggin.