Экзот, вполне может быть, мне самому интересно.
Единственное что более менее вменяемое я нашел это вот:
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In August 1962, however, the proposed transport, by then referred to as program CX-4, was rejected by the Army, which was of the view that it did not represent any "significant advance" over the C-141. On 20 June 1963 the Air Force released another version of the CX-4, which promised the Army what it wanted.[...]
Requests for Proposal for the Heavy Logistics System (CX-HLS), as the CX-X was now known, went to prospective airframe and propulsion contractors on 27 April 1964. The new CX-HLS was equipped with only four engines, instead of six as planned for the defunct CX-4. [...]
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http://www.globalsecurity.org/military/systems/aircraft/c-5-history.htm
В этих отрывках (по контексту) CX-4 упоминается именно как программа. Правда инет как источник сами знаете...