Here's a link to a template that's based on actual engineering calculations and has been used by a number of people successfully.
If you build two of those or modify the design into a double-wide it will be plenty strong enough to support your tanks. The design doesn't appear to resist twisting very well though, which is why it should be skinned in plywood. Virtually any thickness of plywood would be sufficient, even the very thinnest.
If you read through that thread you'll see that a number of people ask about 4x4s and are repeatedly told that it's a bad idea. For vertical members they're just not necessary, and for horizontal members a 2x6 will give you more strength anyway. Worse, they're apparently prone to warping, dunno why. If you follow those plans and use two perpendicular 2x4s at each corner and attach them at a couple of points they'll never warp.
I built something based on those plans, but it's only for a 10g tank, so I used the scrap I had in the barn which happened to be 1x4. I used glue and screws instead of clamping, and the damn thing is strong enough I could jump up and down on it all day and not break it.