Michelle, that does seem a little young to be on a 4 hour EASY, but if your lo can do it, then go for it. The only way to know is to read her cues. There is no sense waking her up, if she's going to continue sleeping, just to ensure she stays on a 3.5 hour EASY. KWIM? And wow, that is a big baby.
So how many feeds is she having in a 24 period? That could be the most telling at this point.
Good job on the EASY! Sounds like you can read her cues well, so the transition to 4 hour should be pretty easy.