Whole wheat milk bread -- success with two loaves, failure with four

So, I've found a recipe for whole wheat milk bread that I particularly like, and I decided I would quadruple the recipe to make four loaves, yesterday. I usually double it, and everything comes out fine. When I tried baking four, though, they initially had fantastic oven spring. But then they collapsed in on themselves. And when I took them out of the oven after forty minutes, they were not done. I couldn't get them done afterwards, either. I kept putting them back in the oven for fifteen minutes at a time, but no such luck. Unbaked and gummy at the bottom. 

The thing is, I've never had this issue when making two loaves. And I'm wondering why.