In mission 14 of the campaign ("The Breaking") there is a beautiful piece of soundtrack combining the original Halo theme and the Halo 4 theme.
The song plays during charging through the Promethean lines until you reach the triple Warden Eternal.
Does anyone know if this song is part of the Original Soundtrack and if so, what the name of the song is?
(MAJOR SPOILERS AHEAD)
The song can be heard here (video already starts at the correct spot). The song goes on for a few minutes.