What you could do is wait for baby to wake then feed. It might be 3:15 or 3:30 between feeds but that's not all bad. Your A time sounds fine for a 6wo and she's feeding, gaining weight, sleeping properly (I presume, since you're waking her), sounds like all is going well. It doesn't have to be a rigid time either 3hr or 4hr, it can be there or thereabouts.