How to connect four different ground planes in the same PCB?
Suppose you need 1 millivolt noise floors for the battery charger.
Suppose the Microcontroller edge rates cause 1GigHertz energy movement,
Suppose the return-path inductance is 10 nanoHenries (about 1cm of wire, or 10cm of GND plane).
Suppose the Microcontroller has 5 volt transitions, into 10pF loads.

How much noise?

5 volts at 1GHz across 10 pF ===?

Z(10pF at 1GHz) == -j15.9 ohms. Call it 16 ohms, in our pretend precision.

Current into the 10pF (that current needing a return path) is 5 volt / 16 ohms,
or 300 milliAmps (peak, peak-peak, RMS --- we will ignore that).

What voltage drop across the Return path (the GND path)?

Z(10nH at 1GHz) == +j63 ohms. Call it 63 ohms.

What voltage drop? V = I * R = I * Z = 0.3 amps * 63 ohms = 18.9 volts.

But we only have 5 volts. Thus this circuit needs to be modeled with the Z(inductor) limiting the current, not just the capacitance limiting current.

What we do learn? this is a very noisy problem, under the assumptions I've give you.