WHAT IT’S ABOUT Country music’s first major “supergroup” was formed in 1985, comprising Willie Nelson, Waylon Jennings, Johnny Cash and Kris Kristofferson. They took their name from a Jimmy Webb song (“Highwayman”), later famously covered by Glen Campbell, about a soul who went through different incarnations. For these four country greats, the Highwaymen was just another career incarnation, so to speak — one that lasted a decade and yielded a couple of albums.

“Friends Till the End” recalls the glory years, with commentary by producers like Don Was, Kristofferson and others. Jennings died in 2002, Cash the following year. This film argues the Highwaymen was a labor of love, and genuine friendship.

Also of note: WNET will air the entire Highwaymen March 1990 concert from Nassau Coliseum at 9 p.m. on June 17.

MY SAY Dating back decades, the “supergroup” was and occasionally still is one of those reliable gimmicks intended to make fans happy and promoters ecstatic. The idea is that the whole really is greater than the sum of its parts — that is, if the “parts” happen to have recently had a couple of underperforming albums, or are noticeably past their prime years. Can’t fill Madison Square Garden alone? Four stars together should do the trick (right)?

In the case of the Highwaymen, absolutely right. The four superstars comprising this one group in fact sold out Nassau Coliseum in March 1990, while outtakes from that performance form the basis of this film’s concert footage. In 1985, these four may not have been at the peak of their immense popularity — which “Friends Till the End” does gently remind viewers — but their supergroup did reinvigorate their fan base, and reminded everyone else why they were national treasures in the first place.

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“Friends Till the End” does tend to overplay its hand, or at least its metaphor. Mount Rushmore is the favorite one, cited any number of times. To wit: The Highwaymen is “kinda the Mount Rushmore deal,” says Toby Keith. Singer/songwriter Marty Stuart decides it was merely “a victory lap.”

In fact, why an “American Masters” on a supergroup? That’s not entirely clear. Of these four, only Nelson has earned his rightful solo “Masters” portrait (in 2002). Whether Kristofferson and Jennings deserve one is a legitimate debate — I think they do — but the fact that Cash is still awaiting his is a legitimate oversight. “Friends Till the End” can’t even begin to redress that.

BOTTOM LINE A pleasure for fans, but this genial film sometimes seems like a minor chapter in four major careers.