Activity time should increase anytime from now til 4-5 months. My ds didn't got to 4 hour EASY until he was around 5 months old. Before that he just couldn't stay awake for 2 hours at a time. I went by his cues and, when I got an inkling that he could stay awake slowly, I started increasing his awake time by 15 min increments every few days.
As for the 45 min napper, that's definitely normal. My dd already does that and she's only 2 months old! You need to work on extending the naps - there's LOTS of helpful info in the Sleep forum. I know because I needed that help quite a few times for ds.
While he was doing the 45 min naps, though, I would shorten his activity time so that he ended up with about 4-5 naps per day to accomodate that issue, so he wasn't as tired and cranky by the end of the day.
HTH