[new but handy]

you'd be better off useing the 4x4's for your lateral supports and 2x4's for the verticals. wood is way stronger verticaly than horizontally. even if you laminated 2 2x4s together for your horizontals it would be plenty strong

Mark: how much deflection is there to those 2x4s? the angle is hard to tell but it looks like about 1/2". Your stand is only holding about 40g of water and it looks fairly warped. I wouldn't put a full tank on that. Even if you moved the sump over so it is centered on that stand you'd be a little better of IMO