Just wanted to say thanks for posting this. I've been having the same issue so I just ordered a 3" belt!
I opened this thread two years ago. Some time after I got that bruise, I switched to a 3" belt, and I never had a problem again. I've been squatting and deadlifting nonstop since.
I think if you have limited space between your lower rib and hip bone, you might need a 3". My rib cage is lower because of a bad case of kyphosis, so a 4" is just too big for me.
Just wanted to say thanks for posting this. I've been having the same issue so I just ordered a 3" belt!