I agree about being honest about it - the truth isn't something bad for a 4 year old to know. Smoking is a habit someone starts, has a hard time stopping and can be very dangerous for them and the people around them.
As a cancer survivor (Hodgkins lymphoma, not lung cancer), I don't know what caused the cells to grow out of control and threaten my life. But, I do get upset when the people who work in my building sit right in front of it (some in the airlock between doors) and smoke not giving me the chance of whether or not I want to smell it or walk through it.
Our son (3.5) knows that smoking is bad, and that Mommy and Daddy will be very mad if he ever starts. We have a friend who smokes, and when our son sees him doing it (which is a whole different argument), he tells him, "Mr. Collin, if you smoke, you will die".
We've chosen to take an honest approach to answering all of his questions beginning with my diagnosis the day before his second birthday. I don't want to shelter them from life's curveballs because they have affected us so profoundly.
Good luck.