I doubt that you would find anything to fit in that small space.
You could do away with the 4-circuit panel in that location, leaving the main breaker or its replacement in place, and put an upgraded load center in the unit itself.

The new panels could be surface mounted if the framing limits the space to the dimensions you listed. Panel A could be at one height, Panel B could be higher or lower if necessary if lateral space is limited.

Randy427, you're probably right, I did quite a bit of searching before posting. But moving load centers to each unit sure would be a painful re-wiring job.

Bus_driver, they're already surface-mounted. There's an enclosure on the back wall, fully weather-proof. I could bust out one of the sides and make it bigger. But I'd rather find the Honey-I-shrunk-the-load-box machine.