Geoff Stenner
May be worth having a scout around the S4 Forum and find Philip Hall’s writings on this. He seems to have had success with M7 conversions, but I don’t know whose wheels were being used.
4-coupled are always a problem ( to me) unless the bogie carries some weight. There is also a side-play problem if curves are tight as the decision has to be made re allowing bogie play or driving axle play (rear on 0-4-4t and front on 4-4-0). A compromise might be keeping the bogie with a fixed centre pivot but allowing some wheel side play, and also allowing some driving axle side play too, so that the side play is spread along the wheelbase. I have done this in S7 on the equivalent of 1:6 crossings.