Three windows works but not the passenger side
Most likely it is the window switch. Does the window work from the driver master panel, or from its own panel, or neither? If you don't hear any "noise" in the door of the window that doesn't work, most likely it is just the switch. You can find one at a junkyard or on amazon for relatively cheap. If its the motor than can be more expensive and more troublesome to replace. I would start with the switch. Best of luck!
Looking for a Used Galant in your area?
CarGurus has 1,553 nationwide Galant listings starting at $1,995.
Search Mitsubishi Galant Questions
Mitsubishi Galant Experts