Re: Voltage drop question
The trouble with simply adding a capacitor is it also discharges into the motor. What you need is a way to allow the battery to power your computer and also charge a reservoir in such a way that it doesn't empty back to the motor circuit. The classic way of doing this is to add a diode in line with the computer supply and add a capacitor after it (on the computer side) but the penalty for doing this is a small voltage drop. In view of being so close to resetting anyway, the drop may actually make matters worse.
Motors draw most current when stalled or when powered but not yet in motion. As the current drops rapidly as they start to move, you may have a slightly different problem than you imagine. It is possible the computer will still function at 4.5V but just for an instant as the motors start, it may be dropping much lower. A voltmeter will not show this, you need to monitor the supply with an oscilloscope to see such a rapid glitch. If you assume for now that the computer resets because of a very short drop in supply, try connecting a capacitor across it's supply (suggest 1000µF) and in series with the power wire to both, connect an inductor of a few mH. DC will still flow through it as though it wasn't there but it will hold back sudden spikes and dips in the battery voltage. No guarantee it will work but for the sake of two components it is worth a try.
Brian.