My baby is 4-months old and she just doesn't sleep at night. Please suggest something.

Have you considered letting baby suckle on a pacifier? My friend had the same problem with her baby who would fuss the whole night through. After introducing the pacifier, things improved, reckon it's because the pacifier helped self-soothe baby. You could try making the room where baby sleeps more conducive (if it is not already) -- blackout curtains, white noise machine and far away from the hustle bustle of everything else.