A lot of Mums have reported their bubs beginning to wake during the night again around the 4 month mark.

The belief I am lead to believe, is that with the 4 month mark they are learning a whole new bag of skills, and their little brains are just in overdrive.
Remember when you used to study for an exam...and when you tried to go to sleep your brain wouldn't shut down...apparently its kinda like that for the babies.

Good luck!!