4x100 meter base to 2x200 without changing meter base?
If each apartment can make it on 125A, I would leave it be.  You need a third service anyway for commons space loads, unless there are none or very few and you are doing something clever like installing a solar/battery system for those. 

Otherwise yes, you will need to replace with a 3-unit 200A meter; two for the apartments and one for the commons loads.  

Commons loads must now be on a separate meter, per many states and municipalities, and of late, NEC as well. 

Even if you can weasel out of having a commons meter, I would still get a spare meter base for one.  The reason is rooftop/carport/onsite solar, whose economics just keep getting better and better.  Once solar shingles reach net parity with regular shingles, nobody will install anything else. 



Note that with this conversion, the former main panels will now be subpanels. Any 3-wire service lines must have ground wires retrofitted.  You must obtain separate ground bars for those panels, and move all ground wires to them and pull the N-G bonds.  Feel free to have 2+ ground bars, they are cheap.  

Honestly, my goal would be to change little more than drywll, and retain my ability to convert the unit back to 4 apartments in the future for when the market changes.  I for one am stunned that you can get more than twice as much for a doubled unit as a single, but that's not your question. 



To mimimize disruption, I would advice having a setup that allows each service to have 


a 200A main breaker
at least six breaker spaces 


four for two 100A breakers to each panel (so you don't have to replace feeder), and 
two or more for things like EVSE.