Has there been any slash or lemon fanfics written in the 4th person perspective? Like... from the perspective of a guy watching two people go at it?

That would rule! I love second degree voyerism!

That would indeed rule. However, what you described is not second degree voyeurism (unless you're descibing a type of crime). A guy watching someone else watch two people go at it would be more appropriate.

"...He watched as Sakura and Ryu undressed and he was able to see her perfect breasts before they were obscured by Ryus hands (that he was also watching) He was probably thinking about how hot she was, and how he wanted to fondle those awesome boobs. He couldn't see what happened next because they were behind some boxes, but that didn't stop him from imagining... "

-an excerpt from my new fanfic, "Guile watches Ryu and Sakura"