This way, with a 4-way switch, will give the following combinations
P1 - bridge humbucker
P2- both humbuckers
P3 - neck humbucker
P4 - bridge north (slug) coil
The bridge north hot (black) is permanently connected to the hot out to the switch. The bridge humbucker is switched on and off by controlling the bridge south ground (green). It is connected in at R-1 and R-2, meaning the full humbucker works in P1 and P2, but not in P3 or P4.
The neck north hot (black) connects to L-2 and L-3 and is only connected in P2 and P3. The neck south ground is permanently connected to the volume pot casing.
So in P1 only the bridge humbucker is connected at both ends, in P2 both humbuckers are connected at both ends, and in P3 only the neck is connected at both ends.
In P4 the bridge north hot (black) is still permanently connected, but the bridge red and white are now both grounded. That means both ends of the south coil (red hot, green ground) are grounded, and the north ground (white) provides the alternative ground to keep the bridge north (slug) coil active.