If that's the real thing (and it certainly looks it) then it's a BS4. Only 10 built, with 7 surviving. If it were mine then I'd be securing it with a bit more than a chain thru' the back wheel. Why? Cos' the last (very scruffy and incomplete) example sold for a cool third of a million in April '16:
https://www.bonhams.com/press_release/21670/
Jeez....
Update:
The bike sold in the Bonhams Auction has some history, which may/may not influence the price paid:
"Following the Four's debut at the Olympia Motor Cycle Show, George Brough's friend Hubert Chantrey rode the show model, as a solo, in the London-Exeter Trial in December '31, an account of which appeared in Motor Cycling (13th January 1932 edition). At the end of the article, Chantrey stated that he had ordered one of the Brough Fours".
https://www.bonhams.com/auctions/23600/lot/296/