I am surprised no one has mentioned the resent research on the effect of too much television, video gaming, etc., on brain development. This link will lead you to some intersting information.
http://www.limitv.org/preschool.htm
This one is even better!
http://www.allprodad.com/6tveffects.asp
Also, google brain development and television. There is a lot of good information out there.
Ditto all the above on violance and a 4-year-old as well. I'm not anti gaming (I enjoy table top and live gaming myself) I just think it has a time and place.