[Oh, he can answer that! In fact, he's pleased he can answer that, in fact adopts an explaining stance here, finger up and everything!]

The four's got to do with a proper battle style! Apparently, most leagues require you to only use four moves in battle, with the idea you'll come up with creative uses for those four. For people more skilled than me, it works well - I've seen Gold do some amazing things using four or less moves in a match.