The
inline-four engine or
straight-four engine is a type of
inline internal combustion four-cylinder engine with all four
cylinders mounted in a straight line, or plane along the
crankcase. The single bank of cylinders may be oriented in either a vertical or an inclined plane with all the
pistons driving a common
crankshaft. Where it is inclined, it is sometimes called a
slant-four. In a specification chart or when an abbreviation is used, an inline-four engine is listed either as
I4 or
L4 (for
longitudinal, to avoid confusion between the digit 1 and the letter I).