Charging Capacity

Section 2.1 of the installation manual says that the generator can provide 250W @ 5800rpm. Section 2.2 says that the maximum current is 22A but there is a mximum temperature of 80 degC allowed on the regulator, so that may be the limiting factor.

Depending on your existing electrical load you can work out how much spare capacity you have. You may also want to monitor the temerpature of your regulator at various stages of flight to see that you are not approaching the 80 degree limit. Alternatively you can buy temperature labels to stick on the regulator that will record the maximum temperature.

If you are only connecting a few GoPro type cameras then I wouldn't worry as the power consumption is less than 3W each.

The short answer is, "Don't even worry about it."

The long answer is...

4 amps is the capability of the chargers on the USB end at 5 volts.  (20 Watts) ...  Not what the actual draw from the Generator will be.

20 watts on the ~14v bus side is less than 1.5 A.

And that is only if what you plug into it is using it to the absolute Max capacity.

A Tablet, Cell phone and 2 GoPro's will be drawing less than 1 amp from the generator.

As was stated earlier, "Don't even worry about it."