So, Books 1-3, she was a child. Yes, so were Rand et al, but she was basically skipping around the world, caught up in events more powerful than herself.

By Book 4, she no longer has that excuse. She considers herself an Active Player, making Adult Decisions. She pretends to be Aes Sedai for so long that she essentially believes it (I love how Moiraine, the ice-queen who cares little for Tower protocol, still displays flashes of annoyance when Egwene calls herself an Aes Sedai).

Basically, Egwene being annoying in Books 1-3 were a little girl being annoying. Sure, that 9 year old might be bossy and throw tantrums, but that doesn't mean she's going to grow up to be an asshole.

Egwene in Book 4 is an annoying, arrogant ADULT, which makes it worse.