It is called 4 Huajai Haeng Koon Kao (Thai: 4 หัวใจแห่งขุนเขา) or 4 Hearts of the Mountains which aired on Channel 3.
4 Huajai Haeng Koon Kao was a special lakorn as it celebrated Channel 3's 40th Anniversary.
The story started when Thiptara or Nam (Kimberly Ann Voltemas) the youngest of the 4 quadruplets of the Adisuan family, met and fell in love with Puwanes (Atichart Chumnanon) the crown prince of the country called Parawat, who Nam believes is a simple worker on his brother Fai's (Nadech Kugimiya) farm.
When Prince Puwanes visits Thailand, he is victim to an assassination attempt organised by the Prime Minister Shadul (Noppon Gomarachun).
During his recovery, Puwanes has to pretend that he lost his memory in order to protect his identity and safety.