I usually set my dif to +/- 1, and measure the temp of the beer instead of the ambient temp. The fridge doesn't run that much. The reason I use +/- 1 is because the thermal mass of 5 gal of liquid is resistant to temp change. The way I look at it is that for every degree you let the temp rise, that's 1 more degree that you have to lower the temp when cooling. With a higher dif, the fridge runs less often, but for longer periods of time. I look at it as 6 of one, half a dozen of the other.
As for your situation, I would control the temp with a dif of 1 or 2. Typically, a kolsch is going to be fermented cooler to limit ester production. A dif of 4 might be too much leeway. If your beer spends much of its time at 67-68F as opposed to 65F, you are going you are going to end up with a much different flavor profile. If you want to use a dif of 4, I would recommend setting your temp to around 63F.