4 GB is enough. Considering you're only running a 32 bit OS with a 32 bit program, you'll never use all that RAM. No more than 2GB, for that matter, unless you have the "3GB Switch" Windows registry hack enabled, in which case you could use up to 3GB of RAM for everything totaled. But Poser itself will never use more than 2 GB.

You can see how much RAM Poser is using in Task Manager. Under the Processes tab it shows "Private Working Set" memory and how much each application is using.