When using the 4x objective lens on a microscope, the coarse adjustment knob should be used initially. The coarse adjustment knob allows for quick and significant changes in the focus, making it easier to locate the specimen at a lower magnification.
Once the specimen is in focus using the 4x lens, you can then switch to the fine adjustment knob for more precise focusing. This method ensures that you do not risk damaging the slide or the microscope, as the 4x objective lens offers a greater working distance that accommodates the use of the coarse adjustment knob without coming too close to the slide.