Crazy idea: Bestech releases a small current BMS (like 8A charge, 8A discharge) whose JST connector, instead of this:
B0 (some models)
B1
B2
B3
B4
B5
B6
B7
B8
B9
B10
for example, it looks like this
P- (some models)
C-
C-
B0
B0
B1
B2
B3
B4
B5
B6
B7
B8
B9
B10
so that the JST cable is the only wire you need to run to the BMS at all. And the conductors are doubled so the 3A rating on the JST connector is enough to do 4A charging directly through it.