My guess is it's a 4 month thing. It is VERY common for 4 month old's sleeping patterns to change (for the worse!). I learnt this after my DS2 started waking frequently o'night and was really hard to settle when he turned 4 months. I have since read heaps of threads on BB started by mums posting the same questions and all their bubs are 4 months at the time of posting!

The good news is that it does pass, it must be a major developmental age. Hang in there, I hope it passes quickly for you .