Evangelion Unit-04 (4号機, "Yongōki") was an Evangelion model developed in the United States. Unit-04 was a test subject for installing an S² Engine into an ordinary Eva, likely by using the one recovered from Shamshel. Unfortunately, there was an accident involving the S² Engine which caused Unit-04, the facility it was in, and everything within an over 80-kilometer diameter to be consumed by a Dirac Sea. While the S² Engine experiment was a failure, use of an S² Engine was later perfected for the Mass Production Evangelions. While Unit-04 is never seen in the series, promotional materials and other appearances make it appear to be a silver version of Evangelion Unit-03.