In order to supply enough current to charge all 4 batteries, the charger needs to connect to a wall adapter with at least 2A current.
Do not use a Multi charger with your computer's USB outlet! It doe's not provide enough power for charging more than one battery at a time.
If you use a wall adapter with less than 2A current, charge only 2 batteries at a time.
Since the charger's LEDs default color is green, when charging with not enough power, the charger may not charge and show green even if the battery is depleted.