i think you may be better off tapping a fused 5 volt regulator (7805 radioshack) off your 12 volt line (google if you want to know how to wire it, plenty of info). then you can use a voltage divider (resistors) to get the 4.5 volts. if the current is more than 1 amp you may need to run a couple of them in parallel and make sure you heatsink them.
i don't think (don't know what exactly you have) that the half volt will be that big of a difference, but just to be safe you can pull the voltage down to the 4.5V. if it has ic's then the half volt might matter?
i would not tie into the computer!
extending the controls will not be a problem as long as the distances are not superlong.
i am curious to how this is going to turn out. please post pics when you work it out.
hope this helped some.