If you have been gluten-free for 4 years then you would not present with dh UNLESS you have been getting cc'd or glutened. You can test the rash by seeing if going low iodine makes it go away. If it does then you could be pretty darn sure you have been getting gluten from somewhere. BTW, the steroids will make dh better but the down side is that when the steroids are withdrawn; the rash comes back with a vengeance!
A biopsy of dh after being gluten-free for 4 years would come back negative. It certainly wouldn't come back positive. Did the doc take it from a clear place adjacent to an active lesion? If he took it on the lesion then he did it wrong. If it was done correctly then that contradicts that you are getting gluten from something EXCEPT for the fact that the steroids (topical or oral) will make the biopsy negative. So the biopsy that was done is completely useless whether it was taken correctly or not. It would take something like 2 months off the steroid creams before it will not affect the biopsy results.