The 4x4's are 3-1/2 which means the top probably finishes to 3", maybe 3-1/4".
A ripped 2x12 gives you 5-1/2", which easily finishes to 4-1/2.
When making mine from 2x12's I used the 5-1/2" to work around edge knots, leaving me with good yield and an easy 4" top.