Since M5 will be a Genesis shape, it will not currently work with Poser.  M4 will have a totally different look compared to M5--so you'll want to have both I'm sure; I've seen some previews of M5 on the Daz forum and he seems to bear more of a resemblance to M3--but the flexibility Genesis figure makes it possible to attain just about any look you want.  I am not ready to give up Poser but the ability to mix all these shapes seamlessly and with the sub-d having clothing that will fit and work with any variation of body shape seamlessly without having to do conversions is a definite boon. 

Try DS 4 and see what you think.  It comes with enough starter content that you can experiment a bit and see if you want to get into it.  I was skeptical myself until I tried it and now I love it.