2 hours of activity should be about right for a 4 month old, but I guess they are all individuals, so go by your feelings too. It may be that your lo is getting the hang of things already
Waking sometimes at 45 minutes - sounds as though he is begining to learn how to get himself back, but still needs more practice. That is very interesting Alicja, that he used to sleep right through after being rocked - I mean if this works for you, then I wouldn't worry too much. I guess it may become a problem later on though, perhaps. Did you used to rock him in your arms, then lay him down and that was it?
45 mins is the length of the average sleep cycle for a baby, at this time, they come out of a deep sleep into a lighter phase and if they have not learnt how to get themselves back off, will wake, it's very common. So you have to teach him how - with pat/shh. Yes dummies do become props, if he can cope without one now, I would wean him off.