The US Army decided it could use the KY-8 voice enctyptor by mounting it in jeeps and other wheeled vehicles where space was not so critical as in aircraft. An attempt was made by NSA to design a ground based, tactical ciphony equipment for the Army called the KY-4, but it didn't pan out. Independently, the Army had tried to develop a tactical voice enctyption device but it was equally unsuccessful. So Army bought a batch of KY-8's and they and the Marines became the principal users even though it was originally designed for aircraft.
In summary, the KY-4 never made it "out the gate".
1) https://www.nsa.gov/public_info/_files/cryptologic_histories/history_comsec.pdf (page 65 of document)
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