Hybrid gas electric drone
Is it possible to work it this way without looking at the current or
  voltage draw by the motors as long as all four of my motor’s Current
  times Voltage is less than 3.5kW ?


Maybe, if you plan on having the motor voltage vary wildly, then you can run the motor constant and the voltage (or RPM if there is no throttling on the motor) will vary with the output of the generator. If the voltage needs to stay constant (which is usually the case with most electronics) need some way to regulate the voltage. 

Normal generators use some kind of controller (mechanical or electronic) to regulate the RPM's to keep the rotation and voltage output constant when the load changes.