The engine computer detected a misfire on cylinder 4 — that cylinder isn't completing a normal combustion event reliably. You'll usually feel a rough idle or hesitation, and the check engine light may flash on heavy throttle. Continued driving with an active misfire can wash down the catalytic converter and turn a $300 plug-and-coil job into a $1,500+ catalyst replacement.
Causes are listed roughly in order of how often they're seen on Canadian independent-shop tickets. Always confirm with a scan-tool freeze-frame and a circuit/sensor check before replacing parts.
