Quick thought, would 4 x 4 for main support be over kill?
no.
however, the weakest link in a wood stand is not the size of the members, but rather, the selection of members, and the hardware you use.
not all 2x4, 4x4, etc, are created equal. #2 DF is pretty standard, but if you get a green one (wet / heavy), they'll start out straight and true, but the drying process will likely warp the wood. try to find dry lumber that's already cured straight, without knots and splinters. When i build stuff outta wood, i spend 1-2 hours at home depot digging through all the sticks on the shelf to find maybe 10. the guys at HD love it when i do that.. >_>
the hardware you choose to screw the wood together makes a huge difference on the shear force of the stand, as much as the number of screws, and location of screws. don't put it too close to the edge, don't over tighten / compress the wood, be careful of veins so that you don't split the wood, etc. drywall screws have pretty shitty shear value vs. deck screws.