there are no upgrade points to go from five to four. you just need to do 10 races, and keep track of them. it's up to your local district rep whether or not a race or two done on one day licenses will count, but I think most will let it slide.
so, my suggestion would be do the one day license, if you find that racing is for you, get the annual and when you submit for your cat4 upgrade, list the race you did on the one day license.