The delegation also met with UN officials in New York last week and were scheduled to talk with the US State Department Friday.
Those arrested were Khat Sovann, 54, Pum Vannak, 53, Pon Soeurn, 34, and an unnamed fourth suspect.
Rights workers estimated between 400 and 1,000 villagers in Broma village, Chlong district, were set to be evicted.
Family members say Chan Samnang died because of exposure to poor living conditions at a relocation site outside the city.
Villagers say they have been pushed off nearly 1,500 hectares of land to make way for the senator’s sugar cane crops.
Joel Brinkley, a professor of journalism, spoke to VOA Khmer recently about his book and possible solutions to Cambodia’s “curse.”
An estimated 2.3 million hectares were given over as concessions to private companies in recent years, Adhoc recently reported.
The 27-minute film follows the work of a leading NGO working to rally support for an end to widespread land grabbing in Cambodia.
The survey questioned 2,000 people in face-to-face responses in November and December last year.
Cambodia held its annual Plow Ceremony on Wednesday, with royal oxen predicting a good year for beans and corn.
In an account published in the English-language daily, the reporters said they had not witnessed the shooting directly.
The rights group Adhoc reported in March an increase in land concessions that ranged from 70 to 99 years.