As many as 80 people were also reported wounded in the restive Nigeria region that has become a centre of ethnic strife in the country.
In Kaduna city, where 10 people were killed inside a Catholic church and 29 wounded, the attacks also led to reprisals by Christian youth against local Muslims.
Following reports of violence, the government imposed a 24-hour curfew to prevent the situation from going out of hand.
In all, two churches were attacked in Kaduna city while another two were bombed in a nearby Zaria city.
Red Cross officials in the city said in a church located in Zaria town, most of those killed or wounded were children.
At least 25 people have been killed in the violence, initial estimates by officials said.
In the first attack, a bomber tried to drive a Honda Accord SUV into an Evangelical Church of West Africa (ECWA) auditorium but was stopped by security men causing his bomb to explode at the entrance, killing some persons and the bomber, Spokesman of Nigerian police Frank Mba said.
Eyewitnesses at another scene of attack said a bomber drove into KC church in Zaria, killing several people including children and students of a nearby polytechnic.
While no group claimed responsibility so far, similar attacks in the past have been blamed on radical Islamic sect Boko Haram.
Kaduna state has previously seen attacks by Boko Haram.
Last Sunday the group attacked two church services, sparking violence which killed seven people. Hundreds have died in its previous attacks on churches.
Boko Haram says it wants Islamic sharia law in place across Nigeria and analysts suggest it is trying to trigger clashes between Christians and Muslims.
A country of 150 million people, Nigeria's population is equally divided between Christians and Muslims.