rosie did this for a while at 4 months (without the 4 am), and then again (with the 4 am) for a couple of weeks before we started her on solids. i think it is most likely a growth spurt.
does she go back to sleep straight away? if she does don't worry about it, if not then you really should bring the timing back a little earlier. I also found with rosie that when she did this it helped if she was actually quite awake for the feed so she took a lot, and also gave her more in the form of expressed milk until she was definitely full, and that helped with the 4ams a bit. (not every night but some which is better than none).
now she's onto solids she's no longer waking for the DF and she's sleeping right through again. I'm toying with dropping the DF soon, but i'm a bit scared!