I've had an almost romantic 4 cycle--

At the time, I dated my ex boyfriends current girlfriend's x-boyfriend (we didn't have sex though, hence the almost).

hmm.... you can help out the current boyfriend by teaching him how to deal with / please your ex, and he can return you the favor.

Shortly after I broke up with my ex-fiancee, he started dating my best friend's girlfriend, and I subsequently started having sex with her boyfriend (ex-boyfriend, by then). It wasn't QUITE a romantic 4-cycle, considering that her ex-boyfriend and I never had a "committed dating relationship," but we were best friends with benefits, as they say, for a number of years.

Also, in the circle of friends I ran with at high-school age, there were lots of sexual 4-cycles and even more non-hetero 3-cycles. I don't think one high school is really enough to make a blanket statement that those things don't happen.