I have four girls too, two RIRs and two Golden SLs and they did the same thing for a while. They are probably just figuring out who's who in the pecking order. Two of mine were kind of confused because one was smarter and one was bigger so it took a while for them to settle down. I'm no expert though, I just know that mine mellowed out after a while
Yep. My four girls are about 9 weeks old & they've been doing the chest-bump thing for weeks. Sometimes they even jump on each other. I think they're just sorting out their pecking order. I wouldn't worry unless they start drawing blood!