1938 Indian 4 was one of several 4 cylinder motorcycles popular at that time.
They were quiet and powerful plus great to add a sidecar to.
Many people thought the engine and gearbox were car ones but were in fact especially designed as motorcycle engines. The only exception I can recall was the England made Brough Superior which used an Austin 7 engine and gearbox along with dual rear wheels.
The Indian 4 came about when Indian took over ACE motorcycles, after its founder William Henderson died in a crash while testing the ACE. William Henderson and his brother Tom founded and developed Henderson motorcycles. The Henderson 4 cylinder was a very sought after bike noted for its speed power and reliability, and was a popular Police motorcycle until it ceased production in 1933.
Have a close look at the engineering of this motorcycle it really is well thought out