Who invented the 4 cylinder engine?

There's no single inventor of the four-cylinder engine. The development was gradual, with several inventors and engineers contributing to its refinement over time. While pinning down one person is impossible, Nicolaus August Otto is often associated with its early development, as his four-stroke engine designs laid much of the groundwork that later led to widespread adoption of four-cylinder configurations. However, many others built upon his work and improved upon the design throughout the late 19th and early 20th centuries.