They have been identified as Wangchuk or Lobsang Wangchuk, Tsultrim, Choephel, and Lhakpa Tsering.
Three of them joined in the nonviolent March 2008 demonstration in Takste County against the Chinese government's repressive policies in Tibet, while one protested in Lhasa at the same time.
They are believed to be serving their jail terms in a prison located in Chushul County near the Tibetan capital of Lhasa.
Lobsang Wangchuk, aged 26, son of Sonam Tsering, comes from the Dechen township in Taktse County. He received a sentence of up to 15 years for his role in the peaceful protests in Taktse last year. He had studied at the Sangag monastery in Dechen and later worked as taxi driver.
Wangchuk's older brother Tsultrim, age 23, and Choephel, 24, were sentenced to two-and-a-half years in jail.
Lhakpa Tsering, age 22, was a painter by profession before he was arrested for participating in the peaceful Lhasa demonstrations. He is serving five years of imprisonment.
The security clampdown by Chinese security forces and police across Tibet since 10 March last year has left more than 220 Tibetans dead and over 1,294 seriously injured. Over 5,600 were arrested, 290 sentenced and more than 1,000 have simply disappeared, according to the Central Tibetan Administration.