Is she breast or bottle fed? This does make a difference as many breastfed babies can never manage a 4h EASY. Most bottle-fed babies can reasonably easily and I don't think a 'cold-turkey' switch is much of an issue. Where I work babies much younger than 4 months are often changed to a 4h feeding routine over just a couple of feeds - so the team work out say a 3.5 hourly amount and give that for the first increased feed, then 3.5h later give a 4 hourly amount, and then you are there
If your LO is able to manage the volumes needed and is happy to wait 4 hours there's no need to transition super-slowly.
That said, the biggest change between the 3 and 4hrly EASY is not really the feeds, it's the awake times. It may be that if you LO can't yet handle around 2h awake that you won't be able to do a straightforward 4h EASY as eat and sleep times will clash. You sometimes have to be a bit creative and it's not unusual to have an AEAS pattern some where in the day. Don't worry if that's the case, just roll with it and it will come right as she gets to handle more time awake x
Let me know if I'm wrong about this connection (baby weight and how long they can go between feeds).
There's some truth in this but every baby is different
