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.