How a Shilka-style system changes 1930s Japanese air defense

What changes if Japan can produce a ZSU-23-4-equivalent anti-aircraft system in 1930?

Low-level attacks become far more costly, pushing enemies toward bombers, rockets, and stand-off weapons sooner.

The scenario matters only if Japan can manufacture and deploy the system in volume rather than possessing a single displaced example. Quad rapid-fire cannon with effective fire control would make low-altitude attack against ships and defended positions far more dangerous. High-altitude bombers would remain outside much of its useful envelope. Opponents would respond by developing rockets and other stand-off weapons earlier, showing how a strong point defense changes attack methods rather than ending air attack.