I think 4 is a good number.
I'd add a restriction as well, such as a maximum of 2 per position. This will keep teams from hoarding all the RBs or WRs, thus making the draft more interesting. This is what I do in my keeper.
Another one I am in there are 4 keepers, but you can only have 2 at any one postion. 2,1,1 or 1,1,1,1. In example, 2 RB, 1 WR, and 1 QB; 2 WR, 1 RB, and 1 QB; or 1 QB, 1 RB, 1 WR, and 1 TE.