I am a collector of J. D. Dougall guns. I have 12 twelve of his guns including a double rifle.
I have also written/published three articles on J. D. Dougall.
I know of two other Dougall 4 bores. Both of these are single-barrel punt guns, i.e. having a an oarlock for a forend.
If your gun is a SxS, it would be extremently unusual.
I do not know how much your gun is worth, but it is of any help, I recently purchased an 8-bore Lockfast SxS.
The asking price was $9K. I bought it for significantly less. This gun is in encellent condition.
If your gun is in good condition, I suspect it is worth signficantly more than my 8-bore If it were mine, I would insure if for somewhere between $12-15K. It might be worth much more to the right person. You have a very neat gun. Treasure it.
This is my first post to this forum. I don't know if there is any way for you to reach me directly, though DuckHunting Chat, but if you can reach me via e-mail, I will send you a copy of an article I wrote on on J. D. Dougall.