The Yamaha 321 does not have a 3rd valve trigger, the King 2280 does. See this link for some pictures:
King 2280
Snorlax,
You're correct in saying that using the left hand to operate the 4th valve on a '4 valve in line' model might cause problems with the valve sticking. You sure wouldn't be pressing the valve down directly as you would if used as intended. Plus using the LH on the 4th valve when standing would be very difficult and still be able to support the horn.
When I move my pinky, my ring finger moves almost as much because of the tendons being shared. This is one reason I looked for a 3+1 setup to start with many years ago.