If you are planning on doing the 4 steps, 1 per day, can you use the toilet if the drain clears on the first application?

Yes, the toilet may be used during the day, or if the drain cleared on the first application. In addition to the first application, just follow the directions for best results to maintain a clear mainline, which indicate this should be used on 4 consecutive days (so 3 more days if you've already used it once) and at a time when the product can sit undisturbed for ~8 hours before flushing (say at night before going to bed).