If you have used PBooost then you should really try it's latest incarnation PBooost Virtuoso - it's library interface is a big improvement over P6's and yet it should still work with P4.

Using tromnek's PRPC you can "1-click" load into Poser, and the library structure is so flexible you can create folders containing, say, everything for 1 figure - figure, pose, face, clothes, etc., and load it altogether!  That's right, anything from any Poser library co-existing in same folder.

And best of all it's still free to PBII owners as Howard's been focussed on other things recently - see his website www.hogsoft.com.

Speed - and dial responsiveness - of P4PP and a good library manager.  Use a stand alone renderer, and there isn't much you'd need P6 for (P5 is a joke, and P7 looks to be trying to fill its shoes so there are realluy only those 2 choices).