There was a question about this before, a few days ago. Yes a 8x video card will work on a 4x mother board. This is the post about this before, it will answer all of your questions.
Originally posted by GameGURU
Yup, It wont be as fast but still will work.
It will be a bit slower but nothing that will be very noticable. ag3 knows what he is talking about, see his posts. What card are you thinking about buying?