Indian 4 Series

Indian purchased the ownership of the name, rights, and production facilities of the Ace Motor Corporation in 1927. Production was moved to Springfield and the motorcycle was marketed as the Indian Ace for one year.

In 1928, the Indian Ace was replaced by the Indian 401, a development of the Ace designed by Arthur O. Lemon, former Chief Engineer at Ace, who was employed by Indian when they bought Ace. The Ace's leading-link forks and central coil spring were replaced by Indian's trailing-link forks and quarter-elliptic leaf spring.

By 1929, the Indian 402 would have a stronger twin-downtube frame based on that of the 101 Scout and a sturdier five-bearing crankshaft than the Ace, which had a three-bearing crankshaft.

1939 Indian 4, in the "World's Fair" color scheme, in commemoration of the 1939 New York World's Fair.

Despite the low demand for luxury motorcycles during the Great Depression, Indian not only continued production of the Four, but continued to develop the motorcycle. One of the less popular versions of the Four was the "upside down" engine on the 1936-1937 models. 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.

In theory, this would improve fuel vaporization, and the new engine was more powerful. However, the new system made the cylinder head, and the rider's inseam, very hot. This, along with an exhaust valve-train that required frequent adjustment, caused sales to drop. The addition of dual carburetors in 1937 did not revive interest. The design was returned to the original configuration in 1938.

Like the Chief, the Four was given large, skirted fenders and plunger rear suspension in 1940. In 1941, the 18-inch wheels of previous models were replaced with 16-inch wheels with balloon tires.

The Indian Four was discontinued in 1942. 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.