I think that just means they won't provide support for Poser versions earlier than P6.

I don't see any reason why V4 wouldn't work in Poser 5. When V4 came out, Poser 5 was only two or three years old. I remember people using her in Poser 4. I'm pretty sure she was actually designed for Poser 4. Her default textures don't take advantage of any of Poser 5's then-new shaders. Or didn't, back then.

I do recommend that you install her in a runtime that's not in Program Files. Poser 5 supports multiple runtimes, so you can put a runtime somewhere else. (Windows security doesn't like runtimes installed in Program Files, which was the default in the Poser 5 days.)

DAZ also has a money back guarantee, so if it doesn't work for some reason, you can get a refund.