Short answer: It would be better to get a new cable than chance it with either of the cables.
Longer answer: While it seems fairly obvious that a 6 volt cable would fry the thing, not giving it enough voltage (as with the 4.5volt cable) could damage it too. Even if the wattage is the same, it's a lower voltage. To achieve the same wattage, it needs to feed in more amperage - wattage is nothing more than volts times amps.
In a storm, that's usually what fries your computer during a power surge - not the initial spike in voltage, but the point where it drops below normal levels before returning to normal. That's when all the extra amps get pulled in to keep the wattage constant, and the amps are what actually damage computer components.