Police said the the four operatives of Jama'atul Mujahideen Bangladesh (JMB) took part in the April 23 murder of Rajshahi University teacher Rezaul Karim.
"Of the four, three were directly involved in the attack while the other was waiting with a motorcycle," Rajshahi metro police commissioner Mohammed Shamsuddin told reporters.
One of the men who was arrested on Sunday from Bogra town has been identified as Maskawat Hasan Sakib alias Abdullah, the "operational commander of JMB for Rajshahi region.
Police said the three others were arrested in Rajshahi city yesterday, but did not disclose their identities for "in the interest of investigation".
"Abdullah took part in the killing. The others, who have been arrested, are also JMB activists. They provided help and logistic support," BD News quoted Shamsuddin as saying.
"The motorcycle used by the attackers at the time of the murder has been seized. A machete and a dagger were also found," he said.
Outlawed JMB has carried out a series of bomb attacks across Bangladesh, killing scores of people including two judges, prompting a massive anti-militant campaign.
Police earlier had said four people, including a leader of Islami Chhatra Shibir, the student front of the Jamaat-e-Islami, had been arrested over the murder.
Suspected Islamists hacked Karim to death using machetes near his house in the northwestern city.
Karim was the second professor of the same university to be killed in nearly identical manner in past two years.
"ISIS' Amaq Agency reported the group's responsibility for killing Rajshahi University professor Rezaul Karim for "calling to atheism" in Bangladesh," it had said in a tweet.
Bangladesh, however, ruled out existence of foreign Islamists outfits like IS and alleged that fundamentalist Jamaat-e-Islami was patronising the killing spree to portray the country as an abode of foreign militants.
(Reopens FGN 18)
In the past two days police arrested three Rohingya Muslims, who took refuge in Bangladesh in view of reported persecution by authorities in neighbouring Myanmar, for their alleged involvement in the murder of a Buddhist monk in southwestern Badarban hill district last week.
Police on Sunday arrested 37-year-old Shariful Islam alias Shihab from Kushtia over the killing of the country's first gay magazine editor and his friend.
Police identified him as a member of outlawed Ansarullah Bangla Team, which according to the Site Intelligence Group was the Bangladesh affiliate of Al-Qaeda in the Indian Subcontinent (AQIS).
Xulhaz Mannan, 35, editor of a gay magazine, and a 25-year-old fellow activist Mahbub Tonoy were hacked to death in an apartment here on April 25 by up to seven attackers carrying machetes and guns.
