There was no immediate claim of responsibility for the blast, but in recent weeks Somali-led jihadists have stepped up roadside bombings.
There was no immediate claim of responsibility for the blast, but in recent weeks Somali-led jihadists have stepped up roadside bombings in border areas, killing dozens, mostly police officers.
The bus was travelling between Lafey and Elwak in Mandera county close to the border with Somalia when an improvised explosive device detonated.
"The vehicle was using a prohibited road which is classified as dangerous because of proximity to the border. We had discouraged vehicles from using this road," he added.
"We have lost four people in the incident while 11 others are injured," said regional commissioner Mohamud Saleh.
Mandera county governor Ali Roba said a local chief was among the dead.
Four people -- three of them aid workers -- died in a roadside bombing close to the Dadaab refugee camp earlier this month, while last month saw a spate of bombings that left 20 police dead and others injured.
Roba blamed Al-Qaeda aligned Shabaab militants for the attack saying on Twitter that it was another example of the group's "default mode of senseless killing" after targeted attacks on Christian civilians in past years had "failed... to divide us along religious lines."