I have a '96 and three '93 Legacy wagons, with a combined 750K miles. The keys are getting worn down plus it sometimes gets confusing in the dark as to which key fits each car. I'm a decent auto mechanic and locksmith for building locks, but have limited experience with automobile locks. What's the best way for me to get all four vehicles to be keyed alike for ignition, driver's door, passenger door, and rear hatch (I probably wouldn't worry about the gloveboxes) and several copies of the key?

Take the cylinders out, to the local locksmith shop. Most can re-key any cylinder you bring in. Pick which key you want to use and have them match the cylinders to that key. Or have them cut a brand new key and match to that.