That charger is a 50watt charger. It will charge your 4S lipos, but prob at a reduced charge rate to stay under its 50watt output power limit. This will extend charging time from a little to a lot depending on capacity of the lipo. You could set the charger up to parallel charge 2 at once, but it would now take twice as long as it would with a single and that's extended charge time to start with due to the 50watt power limit of the charger.

You need a charger with 85-100 watts of output power to charge a 4S with capacity of 5000 to 6000mah.

There are a couple of true dual chargers that have 200watts of power and up to 10amp in each port. The Hitec X2 and Mains Prodigy chargers are made by the same company, and just look a little diff. AMains is a little cheaper and you'll need a power supply to go with it as well. There are no AC/DC powered duals on the market that I'm aware of.

http://www.amainhobbies.com/product_...S-10A-200W-x-2