I'm a serve and volley guy and started at 3.5 when I got back into tennis a few years ago and am 4.0 now.
In singles, at 3.5, it is pretty effective but at 4.0, it really requires at least two of good placement, movement, pace on the serve and then you have to back it up with well executed volleys. I can hang with or beat most "lower" 4.0s but I have to really be on my game to stick with "higher" 4.0s. Their passing shots are just too good if you give them a good swing at it.
In doubles, I agree with other posters. Having a good serve and volley game, or at least getting to the net quickly, is a big advantage at 3.5 and begins to be a must at 4.0 unless you have pinpoint passing shots and great offensive lobs.