Has anyone ever gone on a "field run" during the interview process?

I have had 3 interviews with a company for a sales position, and I have been informed that I've made it to the 4th step, which is a field run with one of the sales associates. I'm basically going to go around with him for the day and see what his day to day activities are. Has anyone done this? What should I expect? Is the job mine to refuse at this point?

Never assume a job is yours until you sign an Employment Agreement. There are always other options the employer can pursue.
A "field run" is used to give you exposure to some of the responsibilities associated with that position. I used the word exposure because you will be exposed to the sales associate, the customers and others. You will be under a microscope in everything you do, so you need to look at this as another interview step.
At the conclusion of your "field run" the company will be asking the sales associate and probably one or two of the customers you called on, how you handled yourself, what your style was, what questions you asked, etc. They will probably also ask what you ate for lunch and if you ordered alcohol. The key question they will ask the associate is,"Can you see yourself working with this person or can you see this person representing our company well"?
Good Luck and go into this as if you were in an office interviewing with the VP Sales.

I believe you got the job. They wouldn't waste your time doing that unless they wanted you. It's possible they want to test out your skills by putting you on the spot too in front of a client (maybe). It's yours.