The biggest culprit for water in the front passenger area is the ventilation flap at the front. The old (pre 1979) ones with a metal flap opening outwards were no problem, but the newer ones have a flap that opens inwards. Even with this flap pushed as tightly closed as possible, my experience is that water still gets in.
I have seen R4s with the front ventilation grill blanked off (on the outside) by covering it with a perspex sheet. No water ingress. No ventilation either, but open windows also work (and when it is raining you wouldn’t want them open anyway).