User blog comment:Mitsuki Kisaragi-San/S2 S2 Song Previews/@comment-29752621-20170310024342/@comment-29752621-20170310031938

Good catch; I forgot about that. Although, "We Wish You a Merry Chirstmas Stars Ver" did have all four of the first years in them, and many of the promos show only Yume, Rola, Mahiru. I think there's four singers because it's the OP, like they did for Star Jet.