A straight-four engine is a four-cylinder piston engine where cylinders are arranged in a line along a common crankshaft.
Diagram of a DOHC straight-four engine
1989-2006 Ford I4 DOHC engine with the cylinder head removed
2006-2009 Nissan M9R diesel engine
1908–1941 Ford Model T engine
A flat-four engine, also known as a horizontally opposed-four engine or boxer engine, is a four-cylinder piston engine with two banks of cylinders lying on opposite sides of a common crankshaft. The most common type of flat-four engine is the boxer-four engine, each pair of opposed pistons moves inwards and outwards at the same time.
2007 ULPower UL260i aircraft engine
1904 Wilson-Pilcher water-cooled engine
1952 Jowett Jupiter water-cooled engine
1955 Porsche 550 Spyder air-cooled engine