One of the reasons (beside comfort and ease of use) 3-point belt is used in cars is that you want the uppe body to be able to move sideways in case the roof is deformed. When using a 4-point your shoulders are fixed and the only thing that can move is your head - a good way to break your neck.
This should not be an issue in monocoque-style aircraft like the Cirrus where it's unlikley the roll-over-bar will deform - don't know about older aircraft though.