My LO couldn't stay awake very long at that age too, and just as you describe, wouldn't be hungry at the 3hr mark. We did a EASAE kind of thing, so that means A time after the nap instead of a feed. Sometimes we were feeding 30 mins prior to her next nap. Don't worry though as long as you are not feeding to sleep you will be okay!
45min naps were par for the course right around 13 weeks as well. It usually means that he is ready for more A time. Try to extend A times in 5-10 min increments over the course of 3 days....you may get a 30 min nap, but the next nap cycle will be long to make up for it. Tracy suggests working with the morning A time, but that was always the shortest with my LO, and the hardest to extend. She had a long 1st nap anyway. Try extending the 2nd A time and you should have some success. Then work your way to the next A time etc......
Don't worry about the "4hr EASY" as a goal. Lots of BF LO's don't make it! Is your LO BF or FF? I was very fixated on this...my LO is 6.5 months and just now is doing a true 4hr with the exception of the bedtime top up!
HTH
Lyndsy