Is there a way to make motion blurs in Poser 4?

No there isnt.

Which is why I hope Curious Labs, the new owners of Poser, make third-party plug in architecture a reality in Poser 5 so we can use things like revisionfx's(?) automatic motion-blur plug in that they make for mediums such as Adobe After Effects.

If I want a motion blur. I make it in photoshop

You can fake a motion trail/blur IF you have patience and a high end system - just use copies of the figure with increasing levels of transparancy trailing out behind the final location. This can take a LOT of processor power and memory to work with though, unless you use the "render scene once with two or three figures, use this as a background for the next render, etc." trick someone pointed out a while back. Then it takes a lot of patience.

easier to do the motion blur in post work, and probably better looking too

I don't know how easy it would have been to manually add motion blur to my recent 5.700-frame Poser animation because I didn't try. But after viewing revisionfx's web site and seeing what their plugin does, I can say that I would use it if it were available for Poser. But more importantly, I think third-party plugin architecture would enhance Poser's potential for creativity to a new level.

I don't mess with animation, but i would think adding a motion blur manually would be an almost impossible task and would take more memory and processor power than most of us have. Poser needs some plugins, that's for sure. A hair and fur plug in would be especially welcomed, as well as motion blur and maybe some volume and particle effects. Bryce could use a particle effects plug in as well.