When you see the live performance of this song.
On the second verse, taeyeon was actually singing along with hyoyeon.
Taeyeon was for the prime vocal and hyoyeon was on the melody.
In beautiful stranger, yuri was luckily given a part and the other 3 (hyoyeon, yoona and sooyoung) was packed for back-ups only.
Honestly, 'twas crucial! Why not give them equal parts as with the other lead vocals?
Snowy wish, my j, wake up, boys and girls, tears, girlfriend, sunflower, oh, run devil run (arguably for yoona but yoona had more screens so it's fair).
Try to listen to these songs. They were given equal parts.