IMO if anyone at this point is still using poser4 it should be their responsibility to learn how to convert newer MATs to it.
this is the only program where people actually expect support for four versions back.. five if you count the new poser pro :(

that said, sometimes i still included a small zip with poser4 MATs if it doesnt take me too long to make them, but i clearly labeled it as unsupported - partly because i dont even have the program installed anymore. 
i wont be doing it anymore however, since i will be using my other file slots for more important content.