'Sky Castle's stars will be on 'Happy Together 4'.

SF9's Chani, Kim Hye Yoon, Kim Bo Ra, and Cho Byung Kyu play the younger cast of JTBC's 'Sky Castle', and they'll be coming on to 'Happy Together 4' to talk about the amazing popularity of the drama, and also to talk about the behind episodes of filming.

The episode will air on the 31st, so make sure to catch it on air.