It’s been 4 days, he hasn’t reviewed or rate my gig. what should I do now?
Nothing. Buyers don’t have to leave a review.
Leaving a review is the prerogative of a buyer.
There have been many forum posts complaining that sellers had got a warning because they asked for a review.
Best just to leave it and move on.
can you tell me, after auto completion the buyer is able to give a review for how many days?
Suggest you read the Terms of Service at the bottom of the Fiverr main page for the rules as to how Fiverr works.
Everything is explained there in great detail.