It has been 4 years since I smoked.
我戒烟4年了。

讲解：
smoke是过去式smoked，所以，这个吸烟的动作已经是过去的。
千万不能理解成“我抽烟4年了”.
我抽烟4年了的说法是 It has been 4 years since I started somking，或者 I've been smoking for 4 years。