They will charge a 4S, but you'll see the charging current limited by the charger to stay under its output power limit. A 4S (minimum 1/8th lipo) is 16.8v and 50watts would limit charge current to 50/16.8 ~= 3amps (2.98a actually).

Its just a matter of power. Heres a more powerful one from hobbypartz. But its a DC only and would need a power supply. With 200watts, it would not be limited even at 6S -

http://www.hobbypartz.com/h610-hotpower-charger.html

There's an AC/DC charger - Dynamite Passport Ultra is 100watts and wouldn't be limuted at 4S, but would be at 5S slightly (4.8amps), but its a $130 buck charger. Generally as power increase, you find DC only chargers.

http://www.horizonhobby.com/Products...ProdID=DYN4064