With a baby that is purely BF and before introduction to solids, I have yet to met a mommy who can successfully get them on a 4 hour schedule. Now that solids are introduced at 6 months, that is when I could move LO to a 4-hour routine. Don't stress out about that. If anything, you should be feeding LO more often because you need to increase your supply and stop worrying about the pumping which is a big PITA!
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Try to take a "nursing holiday" as they call it- let LO nurse as much as possible and do as little as possible for 2-3 days and build up your supply then go back to your routine.
I would avoid "topping off" with formula as this will certainly lead to an insufficient supply (it's all supply&demand, KWIM? so if you "import" you'll never have enough). All of this of course, that your babe is gaining sufficiently!
I think you need to work on your supply for a few days and reassess
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