Simplicity can be divine. Or it can just be simple. Musically speaking, you tend to know what side of that line you're on in any given concert moment, and the fact that Anonymous 4's Friday program at St. Mark's Church found the vocal foursome in both places at various times reminded you how delicate their much-loved art can be.

These hugely successful artists - who have specialized in spare, liturgical medieval music, usually unsullied by accompaniment - appeared here annually thanks to the Philadelphia Chamber Music Society until after 2003, when the group was said to be disbanding. Around that time, though, A4 (as the group is known) took an abrupt turn into American folk music. Those clean, slender voices that lend themselves so well to medieval chant also have a young Joan Baez quality that allowed this transfer to be a complete success. The sound concept has only passing resemblance to the 1960s folk revival; A4 is after the sound of 19th-century backwoods churches, allowing mandolin/violin accompaniments and some hillbilly-ish hiccups into their singing.

Now that A4 is indisputably alive again, the Friday program had one half devoted to a collection of sacred music titled "A Medieval Ladymass," and a second half of newer stuff titled "American Angels." Performances weren't always as fresh as they once were, and it's in those less fresh moments that you noticed just how little there is to this music - particularly the medieval repertoire, with scales and form remote to modern ears - when the words aren't sung with inspirational inner conviction. But when that conviction is there, the music has a directness like nothing else: It's the most basic musical communication, from human voice to human ear.

The second half's folk songs and hymns had familiar titles, such as "Amazing Grace," but that one was heard in alternate guise as a revival song, and though many of the familiar words were there, the usual rhythms and melody were not. In fact, there really wasn't much melody at all, but mostly the same notes vigorously repeated over shifting harmonies. And that made a lot of sense when you consider that revival songs function as much to excite as to inspire. People love classical music because it offers musical thoughts sustained over long spans. Folk songs are the opposite of that. And 10 in a row may be two too many.