Both of mine are made from 2x4 and 2x6, just the standard studs from Lowe's. The 4' was about $50 because of the counter top, probably $30 for it. The 6' was about the same, using 2x6's glued together and screwed down for the top.
All legs are 2x6. I think 4x4 would be overkill unless you're...