The motors, being probably metal canned, should each be capable of restraining their own magnetic field to a level that does not affect their neighbours. Should.
The commoning bus rail connecting cans and shafts should do no harm, but equally might not be doing any good, either.
By + and - I assume you mean motor + and -. The capacitors you mention are a GOOD THING. The capacitors accross the motor + and - are also a GOOD THING.
Putting them 4" away I am less certain about. If it is the only place to mount them, fair enough, but the basic prinpcipal of supression is that the supressor should be as near to the source as possible. If you must have the 4" run, make it a twisted pair, so that any interfering signal has the chance of cancelling out. If the cancelling does not hapen, you will have made a nice little aerial to transmit interference, and this will be assisted by the capacitor, rather than reduced.
Funny subject, interference.