As you stated yourself the Poser4 renderengine is a tad limited..for a decent render you should check them all but the last one that says "Ignore shader trees" which will basically make the engine ignore all materials and shaders made using the Poser6 shadertree which Poser4 doesnt have.

Also mind that you use lighting setups for Poser4 as the Ambient Occlusion and IBL lights wont work in the Poser4 engine.

That said what is your system hardware?..Processor and RAM memory...display adapter?...there might be something else going on which does not allow you to use FireFly.