My boyfriend hasn't smoked in 4 days. he's having trouble with the cravings. how long will these last?

Depends on habit: The more cigarettes you smoke and the longer you've had the habit, the tougher it'll be to quit. Many people try quitting multiple times before they're completely successful. Quitting cold turkey is the most difficult way to go about it. Try doing something else when the cravings hit, like chewing gum or exercising. There are many other options, including nicotine patches, gum, and medication.

Soon: The first week is the worst, and he should be over the hump soon. If he feels at risk of relapsing to smoking, he should consider using one of the many nicotine replacement therapies, such as the patch, lozenges, gum etc. A pharmacist or his physician can explain how to use them properly. Good luck to him and you - stopping smoking is a great step to take for both his health and yours.