Actually, it should be easy. The way I understand it, a T2->T4 transit will clear security at the Bus Gate F51, thereby allowing for airside deposit of the transit pax at T4. Wouldn't the bus in reverse be identical? Walk out of T4 to the waiting lounge, take bus, no security in T2 until your departure gate.
All of this "behind" immigration of course (not entered Singapore).

This is what appears to be the case based on the various plans and discussion I've seen. Of course, if F51 is a non-sterile gate and just dumps people pre-security in T4 then yeah, the reverse journey would be more challenging. Does anybody know definitively which is true? I didn't check out the F51 setup personally as I've been out of T1 and T3 recently so it wasn't convenient.