I have a property that has 4 units that I am turning into 6 small units (efficeincies). The building currently has a 200 amp service. Is that service enough to handle the load or do I have to get a 400 amp service installed.

All depends on the square footage of each apt, and the electric appliances each will have, along with the common area loads.

Have your electrician do a service load calculation to determine the size feeder for each apt and the service entrance size.