K4 – KRYPTO Bluetooth Mullion Keypad & Reader - CDVI’s KRYPTO K4 card reader facilitates high security! This KRYPTO Bluetooth mullion keypad & reader is compatible with the ATRIUM BT mobile application and the KRYPTO Mobile-PASS. The K4 works with the secure protocol of CDVI, the encryption keys guarantee end-to-end encryption between the credential, the reader and the A22K door controller.