Mystery Solved

After going through the music on my computer, album by album, I've identified those damn four notes: Sinead O'Connor's "Black Boys on Mopeds."

If I have anything to say in my defense for not remembering, it's only that the four-note motif occurs only at the end of the song, as a kind of wordless fade-out--and I'm more familiar with the Nields' cover version of the song, which ends differently.