Jan,
I have measured the two lamps used with the M3. They would be the ones to use with two Pilas. The High Output lamp operates at 6.8 volts and 2.48 amps, the Low Output lamp at 7.8 volts and 1.20 amps.
I have also tested the Pila 150s, which would fit in the M3. Two cells drawing 1.20 amps, causes the voltage to drop to 6.9 volts. Now the Pila 168s, which fits in the M4, has 50 percent more capacity, so I would expect the voltage to be about 7.1 volts at 1.20 amps.
The bottom line is: Yes, I agree that the 9 volt lamp will be underdriven in the M4 with 2x168s -- at about 7.1 volts instead of 7.8 volts. My guess is that the HOLA would also be underdriven.
I would like to hear from folks who have used 150s Pilas in their M3s -- as a sanity check on my mathematical gyrations. Do you find the M3 dimmer on Pilas?
Paul