Well, when my son (5 1/2 months) was having a hard time falling asleep, we put him in his swing and it worked like magic. Maybe a couple of nights of you doing this will help her get in some routine. Also, is there a calming bedtime routine in place? With our son, it's bath, lotion (while I sort of massage his little legs/arms), jammies, nursing and then sleep. We did the same with our first son (COLICKY) and it worked. We also have some sort of white noise in their rooms. Now, it's the humidifier. Our older son has soft music (instrumentals) playing.
Is she still eating at night? If so, how many times? If more than once, I would try feeding her a little more during the day. At 4 months, I think even BF babies should be about to go about 6 hours at night (I may be wrong on that). We were lucky and our son has been sleeping through the night since about 10 weeks. BUT, he was a chomper during the day and ate a ton - he is breastfed.
Good luck. Hope it helps.
T.