I'm to a physician, so there might also be a medical answer to your question. I think, however, that it is not wise to include the 10% increment just to one run. If your are running to become fitter and if you also want to increase your stamina, you should vary your runs. This means, that you should have one tempo run, one long run, a recovery run and an interval run. You don't have to, but especially when you run 4 times a week, you should have a recovery run. Your long run should be longer than all the other runs, but shouldn't get too much out of touch with the other runs.
The problem is the strain on your muscles. If you do a tempo run, you put a strain on your muscles, because of the speed of your run. At the end your muscles are producing lactic acid and getting too much tears. Therefore your need rest and the recovery run. Your long run is longer than the other runs, but slower and therefore dos not strain your muscles too much. However, if your long run gets, let say, more than 50% longer than the other runs, your will put a strain on your muscles due to the distance and not the speed. This will eventually lead to injuries.
You can at some weeks just increase your long run by the 10% of your weekly milage, but should keep the distances of law your runs within a certain bandwidth. This will build up your entire strength and provide your with enough basis to run injury free. If you don't do a tempo run or interval run, you might reach a plateau too soon and your long runs will go no further either. Never forget the recovery run if you do more than 3 runs a week.