This is a system with two different Electric supply circuit with common load which is in CB4, in this case, one is the Normal source of power and the other in emergency power like Standby Generator.
If both are supplying the load in parallel operation, the system must be properly synchronize.
If CB3 is the tie breaker, and will operate only when CB! has no loss power, then CB3 must be designed with mechanical and electrical interlocking system to avoid motorization of the smaller power supply and for safety purposes.