Yeah, bunnies of all genders hump to establish dominance once puberty hits. It can get aggressive if two unfixed bunnies can't agree on who's boss. It's different from mating humping, which I'm sure will also start happening soon now that your bunnies are 4 month old. Now is the time to neuter/spay, or you will either have warring bunnies or a very young pregnant bun very soon.