OK, so if it’s not a sin, can you clarify when the fourth commandment applies?
The fourth commandment means different things for adult v minor children.
As a minor child your parents are much more of an authority, so unless they command something sinful, impossible, harmful, you should try to obey without question or argument.
As an adult, you should always treat them in a respectful and loving manner. Don’t take them for granted (eg if they should happen to help you out financially or with child minding or the like, don’t accept more than you need, and make it a point to either pay back any money or do something helpful for them in return as soon as possible).
If you disagree with them, as an adult you have more room to say so, and to act as you think fit, but disagree with love and respect for them - no resentfulness, name calling or anger. Unless it’s something they have a right to a say in. For example, whether to put them in a nursing home - even if you might be paying for it, if they’re of sound enough mind to decide where/when they want to go.
As for smoking - well, doing anything as well-proven to compromise your health as smoking would be skirting along the edges of sin if not actually sinful. Not saying it’s mortal, depends on the circumstances (how much you smoke, whether your health is otherwise good, and so on), but it can come under the heading of ‘gluttony’ - putting too much of a priority on pleasurable things like food (or smoking).