A 4*L (long) is the traditional three-day British Eventing. These are usually scheduled as the first horse inspection on the Wednesday. Then the dressage phase takes place on either the Thursday and Friday, followed by cross country on the Saturday. Sunday morning starts with another horse inspection and then show jumping phase to complete the competition.
A 4* S (Short) has a different format. The competitors will ride their dressage test on the Thursday and Friday then on the Saturday morning the riders will show jump followed by a slightly shortened version of the cross-country course on the Saturday afternoon.
CCI-S eventing competitions are required to qualify for the same level of CCI-L competitions.