Cassandra is given an audience with King Robert as a candidate on his list of potential brides in 4x03. She flirts with him and then leaves abruptly.
She impresses him by easily handling the fake press session he had lined up to test each of the candidates. Robert later remarks to Willow and Queen Helena that she had toyed with them. Willow dislikes her and states that she seemed "cold and calculating." Willow jokingly questions her integrity because she does not know the best noodle house. Helena does not seemed bothered by any of this.
In 4x05, she appears looking to schedule a second date with Robert. She runs into Willow who tries, but fails to tell her that she is actually the top candidate to be his bride. Cassandra offers to make Willow a lady-in-waiting if she puts in a good word for her with the king.
She gifts Robert a smartphone with nude photos of herself and attempts to seduce him whilst they are talking on the phone. Robert is distracted by Willow's speech on television and breaks off contact with her in 4x05. Cassandra later watches the same speech and looks on bitterly.
In 4x06, Robert searches for Willow and runs into a scantily clad Cassandra (who manipulated security into letting her sneak into Robert's room). She accurately guesses his family secrets: that he dislikes Jasper, his falling out with Liam was over a girl, and that Willow does not love him, but may love Liam (this is yet to be determined). She states that one day he will need her and he will call. She promises to see him soon.