I can not say often enough a crate and a den are not the same!

If people are at home and have been persuaded to see a crate this way then fair enough - if you had a basket, rug on the floor or a piece of foam that was the dog's place it would serve the same purpose.
You are both saying that you were at home. The "den" is a safe place for puppies that have an adult, or litter mates for company.
The original question is asking if it's OK to put a pup in a cage for up to 4 hours and leave it every day in the belief that the pup is happy and secure in it's "den".
Again, I have to say, I think not.