Channel 4 in STM32 Advance Timers TIM1 and TIM8
This is a general purpose MCU with general purpose peripherals, such as TIM1 and TIM8. While it might be well suited for the motor control application by having complementary outputs, it doesn't mean this is the only application this timer can be used for. Some application can benefit from having four input capture channels for example. If the fourth channel is not needed in your application, just leave it unused then.