I hit mostly topspin. The spin rate varies a lot. If hitting harder and lower, usually a less spin. If hitting higher over the net rally ball, more spin. But, almost all my FHs have at least a moderate bit of topspin and a majority of my 2 HBHs.
Sometimes slice the backhand for returns, approaches and when stretched. Less frequently slice the FH for returns and when stretched.
My advice is a 4.0 should use topspin for consistency and control. You don't have to use excessive spin. But, I feel much more confident hitting harder if I put topspin on the ball because it helps keep it in the court. It allows you to play aggressively with a safety margin.