How many days does the buyer have to pay before marking the item as unpaid?

While its true that you  have 4 days before reporting and buyer has 4 to respond. buyer can contact seller and ask for more time to pay.  You might get a postive response.  Speaking for myself, I am very flexible with payment IF buyer communicates.  I always contact buyer when payment is not received to inquire if there is a problem.  When I get a response and reasonable request I respond favorably.  If I get NO response, then I have not pity and file for non-payment and strike against buyer.  There are many decent people out there. and there are also many jerks!