rooke & nathan were never a couple. they had sex once when they were drunk. there is a flashback of it in the beginning of 4x15. they sit down on the couch, nathan says, "peyton broke up with me" then brooke says, "i'm drunk" then they go upstairs and have sex. so that was their "reason" for having sex.
Correct me anyone if I'm wrong. I think I remember there was a time when Nathan had a party at Dan's house/beach house and the party goers found a sex tape of Brooke and Nathan. I believe it was a one night thing. I don't think Nathan or Brooke ever dated but I'm not 100% sure.
They never were really "together" but in season 4, everyone discovers that they hooked up one night because they made a sex tape and Nathan forgot about it, and they were having a party and they found it and put it on.
They only hooked up and had sex because Nathan had just been dumped by Peyton, and Brooke was drunk. So they were basically sitting beside each other on the couch, and Nathan says "Peyton dumped me," (or something like that,) and then Brooke says "I'm drunk," and then they look at each other and go upstairs, and then they make a sex tape.
Unless I'm mistaken, they never were a couple. :)
PS: Sorry if somebody's already answered with the same answer, I didn't check and see.