IMO it can be either and I'd suggest looking at the whole routine, what she has done before (ie how long that nap was and if it was reliable), what her sleep cycle has previously shown to be, if she can self settle, if there might be an discomfort (eg teething) bringing her out of light sleep to wakefulness, and age/developmental leaps - 4 months is prime time for the short naps to start so it may even just be developmental.
Would you like to post your EASY? And a bit of background?