The police in western Farah province were attacked on Tuesday while patrolling in Anar Darah district, about 700km west of the capital Kabul, governor Assadullah Falah said.
Farah shares a long border with Iran and usually is considered a quiet province, with most attacks by the ousted Taliban regime taking place in south and southeastern Afghanistan.
"Farah border police were ambushed by terrorists as they were in a patrol and four police were martyred on Tuesday," Falah said.
In a separate incident on Tuesday in the same province, one suspected Taliban fighter was killed, another was wounded and four were arrested with eight assault rifles, Interior Ministry spokesman Lutfullah Mashal said.
The "exchange of fire between police and the enemies of the government" - the term often used by Afghan authorities to describe the Taliban - took place in Bala Buluk district, Mashal said.
Surge in violence
The latest incidents come amid a sudden spike in violence in Afghanistan, much of it targeting the country's fledgling police and army.
Six Afghan National Army (ANA) soldiers were wounded by a roadside bomb in eastern Kunar province on Tuesday, while two US soldiers were wounded in a small arms attack in south-central Uruzgan province.
Six soldiers of the ANA were
wounded in a bomb blast
Meanwhile, an apparent car bomb in the eastern city of Jalalabad on Wednesday killed at least one person.
The powerful blast ripped through a car near the provincial governor's office, killing at least one person, police and a foreign official said.
Police were investigating whether the attack on Wednesday was a "suicide bombing", said Abdul Rehman, the police chief of Nangarhar province.
"A bomb blast in a parked taxi killed one person in the car and destroyed the back half of the vehicle," which was parked close to the governor's headquarters, Rehman added.
The explosion during the city's morning rush hour at 8.30am (0400 GMT) left the person in the taxi "in pieces", the police chief said.
"At this stage we cannot confirm if the victim was himself a suicide bomber or not. We will know more after investigations"
Abdul Rehman, Nangarhar province police chief
"At this stage we cannot confirm if the victim was himself a suicide bomber or not. We will know more after investigations," Rehman said.
A security official from a foreign non-governmental organisation who passed the site minutes after the blast said two civilians were killed. Police were not able to confirm this.
"The bomb blast was 50m away from the governor's house and killed two civilians," the security official, who saw the crater left by the explosion, said on condition of anonymity.
The blast comes hours before US First Lady Laura Bush is due to arrive in Afghanistan, which continues to be wracked by an insurgency led by fighters from the ousted Taliban regime.
She was not due to visit Jalalabad and will stay in the capital Kabul.
Laura Bush (L) will be visiting the
Afghan capital, Kabul
On Tuesday, two US soldiers were wounded in a small arms attack in south-central Uruzgan province, while six Afghan soldiers were wounded in a separate incident.
A bomb blast in eastern Kabul on Monday injured a Canadian man travelling in a diplomatic car along with three other people. Police have arrested a suspect.
No one has claimed responsibility for any of the attacks, however, US officials have warned of an upsurge in violence during the spring as the Taliban regroup after a bitter winter.