I'd recommend a balancer for any 2S or bigger lipo.....there is no way to 'fool' your Onyx into safely charging a 4S, you'll need a new/diff charger.

Get a balancing charger and one with at least 100watts of charger output power, other wise you'll see a less powerful charger limit charging current to stay under its output power limit. For a 50watt charger, all it will charge a 4S lipo at will be from 3.4a - 3.1amps depending on the lipos voltage. The amps are based on lipo voltage going from 14.8v to 16.8v (3.7v/cell to 4.2v/cell max).

AC/DC chargers are usually just 50watt chargers. They get more expensive at 80- 100watts (generally $100+). Consider getting ~200watt DC charger and a decent 250watt power supply. If you're serious about 1/8th you need a good charger.