Firstly the major difference between both versions is that the 8-Day runs twice as long as the 4-Day, bit obvious really. To realize this, the 8-Day drive weight has a pulley, which effectively doubles the drive string length and therefore the runtime, this also means that the weight increases from 2 Kg to 4 Kg.
Both versions are wound by pulling on the winding string, as with a cuckoo clock, the 4-Day has both the drive weight and the winding string on the same arbor. The 8-Day has a separate winding barrel (as indicated by blue arrow), which makes winding quicker and also provides a hanging point for the drive weight string.