I want to change my full term employment into free-lance how can i best convince my employer?

I have been with the same company for 4 years. I did not get promoted during this 4 years and I am now more then ready to hand in my notice. My dream is to start my own business just afraid of not having an income suddenly. My idea was to ask my current employer to let me work free-lance and then build up my own business on the side. How can I convince my employer and how can i discuss a good free lance rate?