From the placard:
The X-4 was developed for the study of flight characteristics of swept
wing, semi-tailess aircraft at transonic (about Mach .85) speeds.
Northrop built two X-4s. The No. 1 aircraft was first flown by
Northrop on December 16, 1948, and the second X-4 made its initial
flight on June 7, 1949. The No. 1 aircraft was grounded after its
10th flight to provide spare parts for the No. 2 aircraft. Northrop's
part of the test program ended on February 17, 1950, with the 20th
flight of the remaining X-4. Although both aircraft were turned over
to the Air Force and then to the National Advisory Committee for
Aeronautics (NACA) in May 1950, only the No. 2 X-4 was used in the
joint USAF/NACA to explore stability problems near the speed of sound.
The program ended in September 1953 with the 102nd and last flight of
this, the No. 2 aircraft, after proving that swept wing aircraft
without horizontal tails were not suitable for transonic flight. Both
aircraft survived the test program without serious incident. The
No. 1 X-4 (S/N 46-676) is displayed at the Air Force Academy. The
No. 2 aircraft (S/N 46-677) was transferred to the Museum shortly
after the program ended. It was restored by the Western Museum of
Flight, Hawthorne, California, and the U.S. Air Force Museum,
Wright-Patterson AFB, Ohio.