As far as I know files created for Poser 4 should work in Poser 6.
I have a lot of stuff written for Poser 4 that works just fine with
my Poser 6.  Most of the DAZ products, for example, were
written for Poser 4 long after Poser 5 was out, and Poser 6
is really just an enhanced version of Poser 5.
I'm not really sure there are any P6 specific files
you could buy.  Most merchants are still trying to make
their products backwards compatable to P4 because
a lot of people still use P4.

What is there that Poser 6 isn't recognizing in your files.
What does it say when you try to load them?
It sounds to me you have some other problems going on
rather than file incompatability.