The court found Thaweechai Wichakham, 39, Sunthorn Phipuannok, 49, Somsri Marit, 40 and Chatchawal Prabbamrung, 45, guilty of launching M79 attack on February 23 2014.
The grenade landed at bus stop in front of Big C Ratchadamri, killing two children and an adult and injured 21 others people.
The court initially sentenced the four to death but commuted the sentence by one third on ground that they confessed during interrogation by police.