I wouldn't worry too much and definately wouldn't introduce formula. My DS went through a very similar period when he was four months, from memory it lasted 2 or 3 weeks. Before then he was the perfect baby, we'd go days without any crying, he's self settle, sleep through the night and then one day it all changed. I remember wondering what happened to my happy little boy, it was like over night it all changed. I think at that time they make the realisation that when they cry/grizzly you respond and so start asserting what they want, and of course there is so much interesting things going on during the day why would they want to sleep? Just work with him and you'll see that over a few weeks it will change. After a couple of weeks DS went back to sleeping through and his feeding just went back to what it was.

Like Taurean said, the wonder weeks really do come and they can be quite hard on Mum, but they do pass. GL