The eventual configuration was rare in terms of contemporary fighter aircraft design, with only the preceding Fokker G.1, the contemporary Focke-Wulf Fw 189 Luftwaffe reconnaissance aircraft, and the later Northrop P-61 Black Widow night fighter having a similar planform (aeronautics). The Lockheed team chose twin booms to accommodate the tail assembly, engines, and turbo-superchargers, with a central nacelle for the pilot and armament. The XP-38 gondola mockup was designed to mount two .50-caliber (12.7 mm) M2 Browning machine guns with 200 rounds per gun (rpg), two .30-caliber (7.62 mm) Brownings with 500 rpg, and a T1 Army Ordnance 23 mm (.90 in) autocannon with a rotary magazine as a substitute for the non-existent 25 mm Hotchkiss et Cie aircraft autocannon specified by Kelsey and Saville. In the YP-38s, a M4 cannon with 15 rounds replaced the T1. The 15 rounds were in three five-round clips, an unsatisfactory arrangement according to Kelsey, and the M9 did not perform reliably in flight. Further armament experiments from March to June 1941 resulted in the P-38E combat configuration of four M2 Browning machine guns, and one Hispano-Suiza HS.404 20 mm (.79 in) autocannon with 150 rounds.
Question: What was replaced in the YP-38s?
Answer: T1