i don't know of any MODERN articles that compare the difference in performance between x4 and x8. however, there is this article published a couple years ago:http://www.tomshardware.com/2004/11/22/sli_is_coming/index.html
as you can see there are minor differences in speed between x4 and x8. however, keep in mind they're testing the 6800 series cards. the performance gap has likely increased quite a bit since then, especially with the latest 79xx series and definitely with the 88xx series.
my advice: don't bother with SLI on a x4 connection.