Two 2-bank chargers. Easier to find a place to mount two smaller than one huge, and if one bank malfunctions you're out of luck with all four when you send it out for repair. Or a three and a one if the TMs are far away from the starting.
I just got a 3 battery onboard charger from MinnKota at Dicks Sporting goods online. No delivery fee, no tax charged and Minn Kota has a nice rebate going on at their web site. Best deal I found so far.
