A 4.0 serve can trouble other 4.0's.
A 4.0 serve dominates a 3.5.
A 4.0 serve kills a 3.0.
A 4.0 serve, done smart, can trouble 4.5 and 5.0's,
A 4.0 serve can be a sitter for a 5.5 thru 7.0, but that would depend how the 4.0 serve is. A lefty hitting wide and then twisting it up around 6' at the baseline might hang in for one more shot.