This is what I got from the Chinese translation (from Baidu) of the text preview for Ep 4:
After finding out that Se Ryung isn't the Princess, Seung Yoo, on the final day, is still seeking everywhere to uncover Se Ryung's real identity. In the end, as deemed suspect for aggravating the princess and is put to trial by MunJong who believes that he is innocent. However, with the court officials have sending letters to have him beheaded, Seung Yoo is facing a life threatening crisis. SeRyung in order to save him, have sneaked into the prison to tell him her real identity.
Prince Suyang finds out that Seryung met Seungyoo all while pretending to be the princess.
Even with Seryung's earnest pleas to save Seungyoo, Suyang is planning to kill him.
Kim Jongseo decides to resign from his position in order to save his son and makes a deal with Prince Suyang; because of that Seungyoo barely makes it out alive.
On the day of Jungjong, who was chosen as the new consort instead of Seungyoo, and Princess Kyunghye's wedding, Munjong collapses; Seungyoo, who had gone far away, quickly makes his way back to Hanyang...