Hi girls. Like MR, I'm convinced there's some developmental thing that goes on at 4 months. It happened in our house too. (And LM, I initially blamed it on an interstate trip as well.)
I think it is pretty individual as to how long it lasts, how to deal with it and what's effective for each baby. Our frequent night-waking didn't resolve til about 7-8 months old, but that's not to say yours wont get better earlier, KWIM? I hope it gets better for you guys really soon.