XM-4 Skycar Similar to the XM-3, the XM-4 was also a small two-passenger saucer-shaped aircraft. Encouraged by his earlier success of the XM-2 and XM-3 construction of this model began in 1970. The XM-4 featured eight Fichtel-Sachs rotary engines which surrounded the passengers in a circular pattern and debuted in 1974.