I'm sure a nap moderator can help you with this! But, we went through this same problem at 12 weeks. In our case, we needed to extend A times. At 4 months, your LO should probably be able to do more like 1hr45 for A times. Those 45 minute naps are just one sleep cycle, and most likely she's just not tired enough to go longer. At 4 months, most babies are also on a 4 hour EASY. At that age, eating every 2-2.5 hours kind of sounds like a snacker, but again I'm sure a moderator can help you with that.
I think in order to extend the naps you are probably going to have to extend A time first. When my LO first had the short naps at 12 weeks, I also thought I was reading his sleep cues really well! He always went down for naps without fussing. However, when I started to extend his A times, I found out that he really could go more like 1hr45! I was putting him down around the same time as you - 1hr15. At 3-4 months, they start fussing when they get bored or need a change of scenery. They are no longer as content to sit and do the same thing for very long. In our case, at least, I was confusing bored fussing (because we'd never had it before!) with being tired fussing. We started extending A times, and along with some nap extention techniques, his naps lengthened out to two 2 hour naps and 1 45 minute cat nap within a week!
Usually when you add A time, you try to go pretty slow. In my case however, once I let him stay up past the time I usually put him down (1hr15) he clearly was able to stay up longer, so I extended A times by 30 minutes over about 2 days. I think most of the time they recommend 5-10 minutes at a time. It depends on what your LO can handle. By 4 months, distraction works pretty good for extending A times. And if she really seems not able to do much, quiet time in their room can work better.
I'm no nap moderator though, so I'll be curious to see what they say! Your situation just sounded SO like ours, I thought I'd let you know what worked for us. Sounds like you have an angel or textbook baby, though, since she's still going down easily for those naps so my guess is she'll take to a new routine really well! You'll probably have to start stretching those feeds a bit too. Do you know how much she takes each time?
