Go to HD or Lowe's and look for KD (Kiln Dried) 2x4's and 4x4's. They are a bit more, but they are kiln dried to a very low moisture content so they should not shrink or warp as wetter lumber (especially pressure treated) would. Stay away from PT because it is very wet, plus it is treated with CCA (a compound containing arsenic). Not something I want in my house, especially if I had small kids running around.
Of course if you want to really over-engineer, go for a couple of the glu-lam beams held up by 4x4 legs. A stand made out of those could hold up your house.
Mike