Original: 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 ) .
Simplified: When listed as an abbreviation , an inline-four engine is listed either as I4 or L4 ( for longitudinal ) . L4 is often used to avoid confusion between the digit 1 and the letter I.