I have both in my 90G and they are best buddy. Of what I have observed Kole don't seem to protect any territory, they are just grazing here and there. Thus, I don't see a problem mixing them in a 4 feet tank. Yellow tang just seem to considere the Kole as a non-tang fish.
However, I remember one guy posting here saying that his Kole was a terror in his tank.
In fact, I had more problem with my sixline wrasse attacking my Kole, but that is another story.
pheel