Most chargers lose around 20%

4s @ 18 amp = 302 watts

Would require a supply of 360 watts or so

360 watts @ 12v would be 30 amps

Some chargers will not get to full power output on a 12v input, they require a higher voltage (varies 15v - 28v)

What charger are you using ?

If its a decent charger it will have a screen that tells you how much power it is using at its input

Cheers