what does it mean agp video card 4x/8x as advertised on e-bay

AGP (Accelerated (advanced) Graphics Port) had a total of four different operational speeds with some odd variations I won't get into. The first version of course ran at 1X. Then the bandwidth was doubled to 2X, then 4X and finally 8X. Graphics cards designed for each version other than 1X were designed to run either at the specified speed or one version back. So, a grapics card with the 4x/8x designation will run at 8X in an 8X slot or at 4X in a 4X slot. The voltages decreased with each new version and that is the primary reason they are not all interchangeable. Look at the link below for more on AGP. http://en.wikipedia.org/wiki/Accele...

if it says 4x/8x and I need 4x then can I use it?

Verify the voltage your motherboard supports and the voltage required for the card. Goin' Fishin' (Some day)