Police acted on the tip-off that the car that exploded in Ankara had been bought from a dealership in Sanliurfa, state-run Anatolia news agency reported. The four suspects were due to be taken to Ankara, the agency said.
Nobody has claimed responsibility yet for the attack that claimed the lives of at least 36 people - the third on the jittery capital in five months.
Health Minister Mehmet Muezzinoglu gave an overall toll of 37, but said this included at least one attacker and possibly two.