I've yet to upgrade from release 3 of poser 5 to release 4.1 due to the negative feedback when it was first released.
I've found release 3 to be stable on my P4 1.3 GH system and the only real complaint is that it takes a long time to load and save.
So my question(s) is(are):
Is release 4.1 stable?
What are the real advantages of Rel 4.1 over Rel 3?
Does 4.1 load faster than Rel 3? (Some of my PZ3 files are more than 160 meg)
What are your experiences with Rel 4.1

I tried SR4 then went back to SR3 because it took forever to launch and relaunch Poser, unlike SR3 which loaded much faster than SR4 (about a minute and a half compared to over seven minutes) during the initial launch and even faster during subsequent launches (i.e., duing the same system-boot session). I may try SR4 again once I've completed the massive animnation project I've been working on for several months.

I think you'll find that SR4 - as in SR4.0 - was the one with the negative feedback. For me SR4 was slower than any other P5 release. SR4.1 however works, and is as fast or faster - in terms of responsiveness (with Content Parasite removed of course) than any prior version of P5. I'm still no P5 fan or user, prefering PPP for it's responsiveness, but I'd still choose SR4.1 than any other P5 edition - and I've tried them all. Only way I can see they could speed up the response time of SR4.1 (to P4 equivalent) would be to put the Paramter Dial back on the main Poser UI panel where it belongs, avoiding the Windows inter-task communication overhead, but I doubt that'll ever happen. (Dunno what was up with SR4, felt like there was something wrong with its build, like a bit of diagnostic code still compiled in or something.)

One of the key fixes in SR4 is the fix for displacement. Personally, I couldn't cope without it. Until you have SR4, displacement mapping is useless since the shadows go the wrong way. According to CL, you can avoid the slow down by uninstalling Poser and then doing a fresh install and patching with the SR4 patch without using any of the other patches in between. I've tried it and it does work. Loading is still slow, but nowhere near as bad.