At 4 months Peter was so close to rolling over. During the fourth month he's sleeping at night wasn't going so well, he never would let me put him down, so my nights were me holding him, which is fine but I had a lot going on and sometimes it's nice to have a break. He also woke up a lot at night to eat so I think he was going through a growth spurt.
At 4 months I introduced him to rice cereal and it took a while to warm up to it.
He also loves getting thrown in the air.