+1 Using full packet works fine for 4.5l
IMHO, Youngs cider yeast makes a very dry cider which requires some aging to be really good. Youngs Ale yeast might be a better choice to have something drinkable by Christmas. Nottingham is another great choice. If you are in the UK, Gervins English Ale is the same strain for half the price. Brupaks Ale is another good UK ale for producing a cider that doesnt need a lot of aging. Best yeast to use depends on juice so YMMV. Good Luck!