Yes, they are useable by people with only P4. The cr2's will load, however...they won't have an icon in their library (PPP saves a .png thumbnail instead of a .rsr) and they'll get a "higher vesion of Poser" error. After they hit okay to the error, the character loads and works fine.

I keep an instantce of a non-PPP version of P4 installed on my machine, just so I can make sure my figures are fully P4 compatible. I just open them up there and resave to the library which gets rid of the version error and saves an .rsr thumbnail.