“If so be that ye have heard him, and have been taught by him, as the truth is in Jesus:”
King James Version (KJV)
4:21 Seeing ye have heard him - Teaching you inwardly by his Spirit. As the truth is in Jesus - According to his own gospel.
Eph 4:21 If so be that ye have heard him, and have been taught by him. This does not express a doubt that they had heard and been taught by Christ. The idea is: If ye have heard, etc., as I know you have, you have been taught the truth in Jesus.