"If vers 4 does what I need it to do right now, there's not much inclination to leave the plastic hair and limited lights behind in favour of fancier tools."

That's a very rational argument. So I'm not quite sure what inclined you to spend the money on Poser6. Curiosity?

That'll no doubt change some day, and I have the program, so I'm ready for the switch. But for now... I'm sticking with 4.

But since you have spent the money on P6, why not use it? It'll do plastic hair and limited lights just as well as P4, since it can host the same content and use the same render engine. In the meantime you'll get used to the expanded UI and can use new features incrementally as and when you might need them. Going from 4 to 6 isn't a "switch" like moving house; it's like having a big extension on your current house that also moves a couple of familiar walls.

If you already have P6, there can only be three sensible reasons for sticking with P4:

1/ Poser4 does something Poser6 cannot do.

2/ New Poser6 features seriously get in the way of your workflow (related to above).

3/ Poser6 is too buggy.

As for curiosity, Poser7 won't be that far off...