Although the charger is rated 6A it is also rated 60watt, so there are 2 maximums!

4s Lipo needs a charging voltage of 16.8 volt, with a maximum of 60 watt a maximum of 3.5A is possible.

Only when charging 2S batteries (8.4v) makes a charging current of 6A possible.

If you do not know how power, current and voltage are related:

power = current x voltage