What was the temperature of your batch when you bottled? Did you mix the sugar into a solution and then mix that in or rack onto the solution before bottling?
At, say, 70F, for a 4-gallon batch, using corn sugar, 4oz will get you to 2.7 volumes of CO2. That's not overly much, I wouldn't think, and seems to be just right for a Kolsch. I've read that normal beer bottles should stand up to around 4 volumes of CO2, assuming no flaws in the glass.
On the other hand, if you took it straight from the fridge and bottled it, it was at a lower temperature. At 40F, and assuming maybe you did put too much sugar in by 0.5oz, you'd be looking at something like 3.5 volumes of CO2 if using corn sugar - closer to the danger zone, especially when they warm up to room temperature and more of that CO2 comes out of solution. Bottle bombs usually come either from infection (you say you ruled that out, probably) or from uneven mixing of the solution so a larger proportion of sugar ended up in some bottles. Knowing what you did and under what conditions will help.
I always use a calculator and my actual bottling volume and temperature to figure out the amount of sugar to use. (BTW, and I may be wrong on this, but it doesn't really matter what kind of beer you started with, you could bottle straight water with yeast and a certain amount of sugar and still get bottle bombs if the CO2 produced is too much for the vessel. Right?)