Indian purchased the ownership of the name, rights, and production facilities of the Ace Motor Corporation in 1927.

While earlier (and later) Fours had inlet-over-exhaust (IOE) cylinder heads with overhead inlet valves and side exhaust valves, the 1936-1937 Indian Four had a unique EOI cylinder head, with the positions reversed.

[1][4] Recognition of the historical significance of the 1940 four-cylinder model was made with an August 2006 United States Postal Service 39-cent stamp issue, part of a four panel set entitled American Motorcycles.

[13] A 1941 model is part of the Smithsonian Motorcycle Collection on display at the National Museum of American History.

Made to order and hand built the machines have an air cooled, 2 valve, 4 cylinder engine displacing 1845cc.