If you want to be able to provide up to 4.0V into those motors from a 4.5V power source, the minimum voltage drop you can achieve in the emitter follower is the important factor. That's why you suggested using a germanium transistor, and that's why I suggested a negative low-dropout regulator as an alternative since the minimum order value on the germanium transistors is prohibitive.
The amount of heat energy dissipated by the emitter follower (or whatever replaces it) is defined by the current through it, multiplied by the voltage across it. Neither of those is affected by its "efficiency". For a given motor speed, you have a certain amount of power that must be dissipated in that part of the circuit. That means that it will be converted to heat energy. You need to make sure the device won't get too hot. A heatsink may be needed.
