the card should work fine on an agp 4x board, the only difference is the amount of data flow to/from the card. There is only like 5% difference in performance if i am not mistaken.
The difference in performance between 4x and 8x is almost unmeasurable in any game. You have to make contrived examples to demonstrate the difference (i.e. if you write a program specifically to test the difference, you will see it, but in ordinary use it won't be noticed).