4.5 days should give you time to get as far east as Jokulsarlon. This will encompass locations such as Skogafoss, Dyrholaey, Vik/Reynisdrangar, Svartifoss/Skaftafell as well as the glacial lagoon and beach at Jokulsarlon. A good map is the 1:400,000 Iceland Road Map from Freytag & Berndt.