The ability to give 4 (cumulative total) female orgasms to one or more girls during 1 sexual experience, i.e. 2 orgasms to 2 girls, 4 orgasms to 1 girl, etc.
Some dude: Yo man, what'd you do last night?
Orgy guy: I had a fourgasm.
Some dude: What's that?
Orgy guy: If you don't know, you're not ready.
When three gay men are having a threesome, and another gay man walks in and says, "Room for EVEN more?" and the three gay men reply, in unison, very flamboyantly, "Fourgasm!"