Great question, Drew. If it stayed at 4 Brix, I would be happy. But, I suspect it will do down a bit and might even go dry. If it does, I'll back sweeten to 4, no more than 5, Brix.
I don't know how other winemakers keep the Brix where they want. Arriving at the desired Brix and then fortifying would mean the bacteria never takes off. Arriving at the desired Brix and adding MLB (strategy I landed on) and fortifying after MLF is complete will certainly call for back sweetening. Not sure how to do the first strategy.