This was after the employee identified as Muraya Mwangi reported that his ex-wife had conspired with the suspects to kill him.
Those arrested include the woman, her sister and two other men. Police say they also recovered six mobile phones from the two men that they were using in planning the mission.
The Sh400,000 had been paid to the men as part of the cash to finish the mission. The suspects will appear in court today to face charges of plot to murder.