8411's
I use two on my Roo, plenty of power.
If you're using four for safety reason's, you have twice the chance of failure, if one fails and the other is fighting it who wins?
Will the stalled servo drain the batt pak?
What position will it fail in?
In short its anybody's guess which is better, servo failure is very uncommon, and if you pay attention they usually give indications before they fail.
Plus two is cheaper.
My .02 worth.
Pete Stapleton