Should I default to higher or lower power when the recommended power is not an option in a microwave?
One way to approach the problem is to ask the question in a slightly different way: how do I generate the same amount of energy in a 750 Watt microwave that would be produced in a 850 Watt oven over four minutes?

Watts times time will give energy, and since we are not in a Chemistry or Physics forum, let's throw units out the window.  850 Watts times 4 minutes will give us 3400 units of energy (W min to be picky).  To get the same amount of heat from a 750 Watt oven, solve the equation 750 * min = 3400 which is 4 minutes 32 seconds.  In the 950 Watt oven that would be 3 minutes 35 seconds.

Since the power button on microwaves simply cycles the oven on and off (e.g. 80% does NOT mean that a 1000 Watt oven is operating at 800 Watt, but that it is only on 80% of the time), then we can include that in the equation as well for more general problems.  So we have 

WATT * time * power = energy


This equation should allow one to determine the time or  % power to use to get the same amount of energy when cooking in an oven with a different power rating than the recipe calls for.