Dr. Greene's Answer
Kids will sometimes go through phases where their infant sleep or child sleep (and your sleep!) is interrupted, but when the transition is sudden and recent, my first thought is, “What might have changed?”
Often there is a short-term discomfort that makes it unpleasant to fall asleep and that breaks the habit of good sleep. At 4 months old, common reasons would include teething and ear infections. (Interrupted sleep is the most reliable symptom of ear infections. Far more so than ear tugging or fever.) If you suspect she is uncomfortable, you will want to treat that discomfort. If you are not sure, you may want to have her checked just so you will know what you are dealing with.
If teething seems to be causing pain, then something like Hyland’s homeopathic teething tablets or Tylenol may relieve the pain and make her less scared of lying down (pain is usually worse at night and when falling asleep).
If discomfort is not a problem, she might be feeling some new separation anxiety (though this usually does not start until 5 months or later) where she is afraid to be alone. Or her developing large muscle skills may leave her not wanting to be down on her back. Kids often go through rough periods of sleep for the week or two before motor milestones.
The approach I used with my own kids (when I was sure it was not pain or discomfort) was to try to put them down at about the same time every day so they would get in a rhythm of feeling drowsy at that time, then to stand by their sides, gently patting them and singing to them, telling them I love them until they fell asleep. I would do anything except lie down with them or feed them. Within a week or two, they had all learned to sleep well–though this needed to be repeated sometimes, such as after an ear infection.
Another approach is to let her sleep with you. Most families throughout history have slept that way–as well as most families around the world today–and it is a fine choice for some families and not for others.