4 week old “pullet” mounting other chicks...

[Rooster Giddens]

So the last time we ended up with an accidental roo, he was kind enough to start crowing at 4 weeks old, which kept us from playing the guessing game, but this is new... Two weeks ago, we bought 4 chicks (3 that were less than a week old and one older (two week old) ameraucana (assumed Easter egger) sexed “pullet”), and immediately “she” stepped in acting like the perfect baby sitter watching over the little ones (we even named her Mary Poppins). However, this past weekend when we were cleaning out the brooder, we noticed that “she” is mounting the younger (now two week old) chicks. “Her” comb has also started turning red... has anyone else ever seen this (a pullet or cockerel mounting other chicks at such a young age). We have a small flock, and our other rooster is an OE bantam, so if this is a rooster, we definitely need to rehome him.
Also please excuse the mess in the pictures because we have the brooder in our storage room where we are planning for a garage sale.

I'm no expert but 4 weeks old is a bit young for a rooster to begin mounting other chickens? Maybe it's trying to establish a pecking order?