They are among the 11 persons who have been arrested in connection with the killing of S P Shuhaib on February 12.
Akash Thillankeri, T K Askar, K Akhil and Deep Chand were expelled for violating party norms, party sources said.
The 29-year-old youth congress activist was hacked to death at Therur near Mattanur in the district.
The Congress has demanded a CBI probe into the killing.
Opposition leader in the Kerala assembly, Ramesh Chennithala has urged the chief minister to relinquish the Home portfolio alleging that he had not allowed the police to function independently in the state.
The Kerala High Court has ordered a CBI probe into the killing after rejecting the state government's contention that all the suspects had been arrested.