Mohamed Abdulkarim, a security official in the town of Beledweyne, 300 kilometres north of the capital Mogadishu, said the bomber "detonated himself at a tea shop in front of the regional headquarters and four people died, all of them civilians."
Thirteen others were wounded, he told AFP.
Witness Osman Adan said there was a "huge blast... There were several dead bodies and wounded people strewn in the area."
Shabaab militants claimed today's attack, which the group said had killed six "soldiers", including three serviceman from neighbouring Djibouti.
The Shabaab force was pushed out of the capital in August 2011 and lost most of its other bastions in Somalia.
But they still control vast swathes of countryside, from where they launch guerilla operations and suicide attacks, against the capital and against local and international military bases.