I have a feeling you are measuring the wrong way...
You have the black cable from the multimeter at ground for the first cell and measures 4Volt on the second black wire. Then you move the red multimeter-cable to the red wire and measure 8Volt, which for sure is the total voltage of cell 1 and 2. Indicating both cells are at 4 volt and everything is OK.

I suppose (but might be wrong!!!) that the first black wire is ground on cell 1, the second black wire is + on cell 1 and ground on cell 2. The red wire is + on cell 2.