Poser 4 does tend to ignore the parts it doesn't understand. But it's not a bad idea to pull the P5-specific data out anyway, if it isn't being used. It'll result in smaller file sizes, at the very least. Download a few items from Free Stuff and study them to see what does and doesn't belong in a P4 file.

If you decide to edit the files, remember to change the version numbers so no one gets that annoying "newer version than expected" message.

You can convert P5's .png thumbnails to P4's proprietary .rsr format with yarp's P3dO Explorer utility (available in Free Stuff and at SENO Software).