Nebraska coach Bo Pelini confirmed Witt's decision on Sunday.
Pelini said Witt, who will be a sophomore next season, decided it was in his best interest to continue his college career at another school.
Witt, from Wylie, Texas, appeared in five games last season and went 6-for-8 for 48 yards. He also rushed four times for 20 yards and a touchdown.
Lee will be a junior next fall. Spano will be a redshirt freshman, and Green will be a freshman.
Lee appeared in two games last season and was 1-for-2 for 5 yards. He also rushed twice for 17 yards.