Rights groups said they believed foul play was behind the deaths of the three and demanded an investigation into that and other allegations of police corruption and heavy-handed tactics.
The four defendants - Frederick Leliman, Leonard Maina, Stephen Morogo and Silvia Wanjohi - pleaded not guilty on Monday, court documents showed.
They are accused of killing rights lawyer Willie Kimani and his client Josephat Mwendwa, and their taxi driver Joseph Muiruri on June 23. The three disappeared after Kimani and Mwendwa made a court appearance.
Kimani and Mwendwa had filed a complaint alleging that Mwendwa had been shot and injured by police in April.
After lodging the complaint, Mwendwa was charged with a range of offences, including possessing drugs and resisting arrest, according to rights activists.
The four police officers were remanded in custody until Aug. 16 when they are to reappear in court and allowed to apply for bail.
The East African country's police force denies being involved in extra-judicial killings, saying it investigates and prosecutes any officers suspected of breaking the law.
Kimani worked for the U.S.-based International Justice Mission rights group and Muiruri was one of its trusted drivers.
About 300 people marched through the Kenyan capital Nairobi on July 4 to protest over the disappearance and deaths of Kimani, Mwendwa and Muiruri, and demanded that Interior Minister Joseph Nkaissery resign.