4 escs in differential is the same as two, just treat 2 motors (per each side) as two motors. Wire them as you normally would for differential, with splitters instead of direct ESC connections.
With 4 escs, you would be best off wiring all four batteries in paralell, which would give 4x the capacity and possible amp draw for the same voltage.
There isn't really a minimum wingspan for a set weight, as there are many factors that contribute to the total surface area of the wing (camber, chord length, height, ect.) but there are standards for wing loading. As a general rule, the lighter the plane per given surface area, the better (or vice-versa, more wing area for the same weight). Light planes tend to have gentler stall characteristics and can fly slower, a common trait of trainers. Aim for ~15-25 oz/sq. ft for general to aerobatic flying.
As far as actual wingspan, smaller wingspan gives less surface area to rotate through the air, and results in a snappier and higher roll rate, and example being
The Guinea Pig vs. The Pun Jet.
Glad I could help
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