If you just got to 3 hour schedule you don't need to rush to 4 hours. Especially if you are comfortable with that. I was nursing Anna every 2-2.5 hours till she was 4.5 months old and then it was 3 hours, sometimes 4 and sometimes 2.5 again. There is no rule or age when a baby needs to move on unless you see she is ready. In my experience, breastfed babies do better with a 4 hour schedule when they are on solids and nap better. It is difficult to have 45 minutes naps and work them into a 4 hour routine.
So, yes, if she naps well and is able to go longer, you may try but keep in mind that moving to 4 hours will eventually decrease your milk supply which is normal as you have lesser demand. As I said before, when solids are introduced your lower milk supply is natural and not a problem.