Li Ion cells are easily damaged if they drop below 3 volts. Run them flat and they lose most of their capacity and instead of being a 1500 mAH capacity, they drop to maybe 1/4 of that capacity. In a series string of 4 cells I suggest you need 4 analog channels on your PIC, say a PIC16F88. You can measure the first cell directly but you need voltage dividers off cells 2, 3 and 4 to ground so you can measure V1, V2, V3 and V4.

V1 is the first LiIon cell and must not be allowed to fall below 3 V. V2-V1 gives you the voltage of the second cell. V3-V2 gives you the voltage for the third cell, etc.

The PIC16F88 needs no crystal and would be a very cheap solution for up to 7 series cells.

HTH
Brian