07/31/2009. Remarks by Walter van Tilborg: "Edward C. Hoffman built the X-1 in 1960, the X-2 in 1962, and the X-3 in 1976.

The X-4 was built by Hoffman and his son Ed Jr. in 1980 and was an amphibious development of the earlier X-2 two-seat flying boat, but also slightly larger with side-by-side seating. The 115 hp Lycoming O-235 powered X-4 was kept active well into the 2000s by Ed Jr."