4 units 400 amps split from 6 gang meter box
600a is crazy large even if all electric if your current load with 3 is less than 200. if you have gas I would not use anything larger than a 30/40,  300 amp continuous with peaks to 400. This service is much cheaper and would fulfill the code requirements. If all electric I don’t know if I would go over 400 because the cost of the service. I would have each house have there own service drop, this eliminates legal issues you will run into if renting unless you provide power as part of the rent. If you plan on selling later there could also be issues there. 6 gang meters would not be diy and would be off topic. Your service only needs to be as large as the calculated load, multi family dwellings like this you don’t just add 100 for each dwelling you do load calculations per article 220. I would be using aluminum feeder wire, I am surprised you did not require a plan review, I have been out of Ca.  For close to 40 years but thought we had to have reviews back then but that was northern ca (Sonoma, Mendocino and San Francisco county’s). I may be getting mixed up on locations. But you should have the load calculations completed prior to doing anything unless your wallet is large and you don’t care.