I'm wondering how much RAM you have assigned to Poser 4 - click the p4 application icon, hit open-apple + "I" and then open the memory tab. Panther needs 128MB so on a 256MB machine I'd try setting the Preferred Size field to 120000 (120mb) or maybe a little less. It may be that you're still set for the p4 default of 70MB. That extra 50mb could make a difference.
You can also track how much RAM is left by opening the System Prefs., clicking the OS9 icon and clicking the Memory/Versions tab (above).
It'd still be better to get more RAM.
HTH;
Dan