NEW YORK - As with many musical retirements, reports of Anonymous 4's was greatly exaggerated, amounting to a 2004 hiatus that just didn't stick.

The public wasn't willing to let go of the four-voiced medieval group, either as a recording entity or a trance-inducing concert presence. A change of artistic direction happily threw the singers into old-time Americana. And the increasingly numerous stories about their music's medicinal value became a further inducement to limit the hiatus.

"A couple weeks ago, a lady came up and said her mother had recently died and in her last hours, they only played Anonymous 4," said group member Susan Hellauer. "I get emotional just thinking about that."

Ethereal, simple, and gently insinuating, Anonymous 4 recordings are reportedly played during childbirth, for comatose accident victims, and, under less-fraught circumstances, at Christmas - when the group gives its annual Philadelphia Chamber Music Society concert, this year at 8 p.m. Friday at Holy Trinity Church on Rittenhouse Square. This is the group's 10th appearance since 1998 (it was absent from 2004 to 2006).

Next year, the quartet celebrates its 25th anniversary by premiering a work by cutting-edge composer David Lang. Ancient-music specialists often seek out living composers, but few cross musical centuries as easily as Anonymous 4 - or repeatedly rewrite the recording industry's notions of commercial viability. During an hour in Hellauer's Upper West Side apartment last week, she and colleague Marsha Genensky couldn't always explain why they keep flourishing. They do what feels right, sometimes casually, sometimes after long consideration.

On Anonymous 4's Christmas associations: "People want to hear . . . any music that's a cappella, medieval, or Renaissance," says Hellauer.

On their music's spiritual power: "I can't say that we share the same faith in the way that it existed back in the day . . . but we can feel the power of the faith," says Genensky. "It may not sound the way someone heard it in 1300, but the power will come out."

"I sometimes rewrite the words," says Hellauer. "The average medieval Christian was anti-Semitic."

On their foray into American folk music: "I wasn't involved with folk music on any level except that I loved Simon & Garfunkel," says Hellauer. "But I was listening to public radio . . . something on traditional American music, and all I could hear was the harmonies of medieval music."

That last part requires an explanation - and not a short one. Anonymous 4's medieval music discs regularly hung at the top of the Billboard magazine sales charts: The 22 titles (most on Harmonia Mundi) collectively sold an estimated two million copies - reflecting a modern musical appetite that made Chant, sung by Spanish monks, an international hit in the 1990s.

The Anonymous 4 repertoire is mainly drawn from the period A.D. 1000 to 1400. Its smallish scale lends itself to the group's lineup and also accommodates the practicalities of a modern career (only one rental car is needed for out-of-town dates). Since the music survives in notation clearly outlining the melody, its lack of any specific range allows it to be tailored to Anonymous 4 strengths.

"We think of the music as heightened speech," says Genensky. And its creation happened in reverse: The music evolved first through oral tradition and then was written down. Performing it is tougher than it seems, if only because a concert requires careful assemblage and sequencing to maintain a modern sense of continuity.

After more than 15 years of 75 annual concerts, the four longed for a break. Nobody said "retirement," but "a hiatus of unknown length" was planned after a full season of goodbye-for-now concerts.

With less to lose, the group tried out a Genensky-devised program, reexploring the 19th-century American folk music she had studied as a Penn student years before. That turned into the group's 2004 American Angels disc.

Before the CD release, the program was heard at Princeton's Richardson Auditorium - with little advance warning that it wouldn't be typical Anonymous 4. The group was dramatically transformed, but in ways that made perfect sense. The clean, nearly vibratoless voices recalled a young (if not so earnest) Joan Baez.

Genensky likes that comparison: "When I was growing up, I thought I should have come of age at the same time as Joan Baez."

Not only did medieval fans follow them, but American Angels became the group's best- selling disc, No. 1 on the Billboard classical charts. That cut short the hiatus, though the concert schedule is down to 20 to 30 a year.

The two sides of the group's character come together on its new disc, The Cherry Tree (now No. 3 on the Billboard classical charts). It's based on a Nativity legend that Joseph doubted Mary's virtue, spitefully telling her to have the father of her child pick cherries for her - only to have the cherry tree bend down to her, indicating that the fruit of her womb was indeed divine. Much of the disc's music is Old World, but the legend made its way to the New World, and the American version is the disc's centerpiece.

How all this religiosity and angel music connects with the group's inner lives is hard to say. As individuals, the members of Anonymous 4 are exceedingly private. They are neither shamans nor druids. One owns a cat named Rachmaninoff. They're vegetarians - sometimes. They enjoy wine after concerts, but unlike many in their field don't start at intermission.

Hellauer has an extremely young grandson. But the members generally not only withhold their ages but also skirt facts that will allow you to do the math.

Genensky commutes from the Bay Area during rehearsal periods; the other two, Ruth Cunningham and Jacqueline Horner-Kwiatek, live in the New York area.

Even the group's famous vocal blend, unlike anything in all of music, has no obvious source. "It has nothing to do with making our voices sound alike. It's the interpretation of the line," says Hellauer. "If we agree on rhetoric, weight of the syllables, gestures, and shape, we'll sound blended."

"It's what we call unity of intent," says Genensky. "It also has to do with agreeing on what the vowels should sound like."

And vowels vary radically from the Old to the New World. What will it be on Friday? We'll find out - anon.

Contact music critic David Patrick Stearns at dstearns@phillynews.com.