CO-4 — Modifier Inconsistent with Procedure

CO-4 means the modifier on the claim does not match the procedure code billed. In behavioral health, this most often occurs when the -25 modifier is missing from an E/M code billed on the same day as psychotherapy, or when a telehealth modifier is misapplied. CO-4 is an administrative denial, not a medical necessity denial — fix the modifier and resubmit as a corrected claim unless the modifier was correct, in which case appeal with CPT documentation.