You'd be lucky to get those good long naps I think. My DS2 is 12 weeks today and we have just short-napped our way through the day.
Basically 30-40 mins is one sleep cycle; if your baby can join two sleep cycles together then you end up getting that long nap.
4 weeks is extremely early days and I wouldn't worry too much about a routine yet, as everything will change very rapidly. All we have done so far is establish a pattern of eat, play, sleep. I have had him in the sling for the majority of his morning naps and am starting to see them lengthen on their own. Naps elsewhere are trickier, but given that we used to get at least one skipped nap a day I think it's progress. Just wanted to give some context!