The Chosun Ilbo tells us what makes her sexy, though it apparently works to make lots people jealous:
Chae Yeon has a lovely nose as well as overall sex appeal. It is the mole, or beauty spot, that makes her nose look so cute. Her sexy choreography is as hot as Lee Hyo-lee’s.
The same article also tells us what makes Seo In-young so special:
Seo In-young maximizes her strong point with a fashion style that highlights her waist, hips and crotch. At 1.62 m tall, she is the smallest of the four, but like IVY’s, her legs make up for it on stage.
LMFAO. Seo In-young, who I hadn't even heard of until about four minutes ago, is the third-place person I alluded to above. She and Chae-yeon appeared together in April on a show called "Musicbank." They used lots of rain in their performance, and had to do it because apparently the original version was too risque for prudish South Korean sensibilities. Pop Seoul has the pics here, but they're not really safe for work.
Seo In-young is also famous for pole dancing.
* Edit: Now I remember Seo In-young. She came out with this irritating song a year or two ago.