I would think that unless you really need to reduce weight, there wouldn't be much reason to use a 4 cell pack As mentioned before, servo speed and power increase with the extra cell. The biggest problem with this type of cell chemistry, is the length of time it takes to charge them...usually overnight. With LiPo, and A123 type of cells, charge time is about a half hour. That is nice when you make a last minute decision to go flying.
On my larger gas planes, I like to use dual A123 cells, a Wolverine dual battery switch, and the Servo City "Servo Board". This setup isn't the cheapest way to go, but it is a good solid combination that gives me confidence in my power system. I like overkill. I use EC3 connectors for the battery to switch connection, and all other connections to the Wolverine switch and the Servo Board are soldered 16 or 18 gauge wire.
https://www.servocity.com/html/servo...l#.VIIsvWea-kg