I hold a labour relations advisor's diploma, and I've rarely held the same job for a number of years. I've been a business executive on occasion, and it was the same thing. Moreover, I had a job offer from Iron Ore, in your corner of the country, which I turned down because I didn't feel I was able to work for a long time in the same field.
I wonder whether you too aren't that kind of person, because you've never worked in the same place for more than four years. There are people like that. After two or three years, since they evolve quite quickly, they may think that their work is advanced enough for them to be able to go and work in another area. Others are too nice with their employees and, after four years, they feel they won't have as much control and that they would be better advised to leave room for others.
I wonder what category you fall into. To date, your employees see quite satisfied to work with you. It's up to you to tell us whether you would like to change fields in order to be useful elsewhere.