How can I use a BMS to charge 4 LC18650s in series?
Almost certainly [tm]:
The 4S battery string has 0V at most negative and +16.8v at most positive end.
Total of 5 connection points - ground/negative and top of 1st 2nd 3rd 4th cell.
Connect most negative to 0V,
1st top to 4.2V,
2nd to 8.4V etc.
There is a very very very high chance of this being correct.