You could get 2 2-Channel chargers that are capable of 20+ Amps per channel. This would allow you to balance charge 4 battery packs at different levels.

Which charger to get is a matter of budget and desired features.

I use the iCharger 308DUO with a 850 Watt power supply. I run 2S ~5000mAh battery packs and usually Balance Charge at 15-20 Amps.

I've never seen the need for another charger. However, I do race with 6 packs already charged to a high storage level.