What I ended up doing with Josie when I couldn't get naps extended for whatever reason was to give her more of those short naps per day. Seriously, she was taking like 4-6 naps daily from 2-1/2 to 3-1/2 months, because the longer A time was only getting her really cranky.
It wasn't till we got her reflux sorted and the consult with Tracy that things really settled. There were times I'd take her shopping, let her fall asleep in the car, and keep driving past the 45-minute mark so she would get ONE decent nap in during the day. Oh, and she did take longer naps once or twice when she was in the car seat carrier, even if we weren't going anywhere - again, that was a function of her reflux.
Come to think of it, might reflux be a factor here?
Many parents of short nappers report that their babies begin taking longer naps again around 5-6 months.
Oh the 4-hour feed? Doesn't have to be 4 hours on the dot. If the naps are still short, the latest you'll ever be is 45 minutes, and you can tweak subsequent feeds if you are late for one or two. Had to do that as well - J's so-called 3-hour EASY was anywhere between 3 and 4 hours on any given day depending in the nap situation, even once we started solids (which we did at 4 months to try to get heavier food into her to help with the reflux - in hindsight probably wouldn't do it again, but we did, and wow, was that a scheduling "adventure!").