Savioa:
The VZ-4 was tested for a year at Edwards AFB, then given to NASA in 1959. They did some tests, found some undesirable flight characteristics which may have been correctable with technology that would have been invented in years to come. It never did demonstrate its promised performance, I believe. Douglas liked the concept and bought the rights, and proposed an improved version to the US Army in 1961, but there was little interest. The company that seemed to go furthest with the concept was, ironically, Bell with their X-22.
Since they were also flying their XV-3 at the time, they could compare the two technologies and decided that Tilt-Rotor had more promise and practicality. Since then no one seems to have pursued the concept.
The VZ-4 is on display at the U.S. Army Transportation Museum located at Fort Eustis, Virginia. The X-22 is said to be going on display at the Niagra Aerospace Museum in New York, opening in May, and the XV-3 is at the Air Force Museum at Dayton, Ohio