A study combining accurate quantum chemistry and full-dimensional quantum dynamics is presented to confirm the existence of an ultrafast radiationless decay channel from the charge-transfer state to the locally excited state in 4-aminobenzonitrile. This intramolecular charge-transfer pathway proceeds through a newly found planar conical intersection, and it is shown to be more efficient in the presence of acetonitrile than in the gas phase. Our results are consistent with recent experimental observations.