If you're working on neglected paint and trying to do correction work with a one-step cleaner/wax, also called an AIO, then you want to use the high speed setting on a DA like 5-6 with a polishing pad. You can use a cutting pad but it might leave DA Haze.
If the paint is in good shape and you're using the cleaner/wax as a maintenance wax then use the 4 setting.
See all of these articles and I cover this in my how-to book also...