After 20 yrs of smoking i am trying to quit. i am on day 4 without a cigarette. how long does it take for all the nicotine to be gone?

It's hard to be sure: Nicotine has relatively short half-life (30-min-120min) because it is broken-down by liver via oxidation process and thus disappears rather quickly--hence people feel need to smoke often. Chronic smokers will have nicotine in body for longer, but i would guess after 1 wk, there isn't much left to bother you. Traces of that can still be detected up to 2 months later. Congrats on quiting. Good luck.