There is no 'Gospel Harmony'. The Gospels are like a Corinthian cacophony! Each sings beautifully in its own way, but Matthew, Mark, Luke and John as a group will always require a 'Paul' to bring their four songs decently into order. (Historically OR Theologically)
Singing four songs at once - or splicing parts of each song into one Frankensteinian medley - will not produce 'Harmony'. Rather, for any beneficial harmonization to take place, a new song must be sung - one based on the four Greater Songs, that does not merely copy them, or attempt to one-up them, but to bring homage.
The Gospels are ultimate classics. Respect the classics. Play, replay and cover them!
Bill Heroman, Amateur Academic