It's a fourth A-level and I'd say your other subjects are strong, so just pick whichever you think you'll enjoy more and get the better grade in. I can't tell you anything about Sociology because it wasn't offered at my school, but A-level English Lit is substantially more difficult than GCSE. Have you looked at the course content for both? If not, it may help you decide. Also, compare how you'll be assessed for each subject, e.g. how much of your mark depends on coursework and the structure of the exams. I did AQA English Lit A, which was completely different from GCSE - the emphasis was on close reading of unseen extracts, which some people enjoyed but others really struggled with. If possible, talk to the A-level teachers for both subjects and get as much information as possible before you decide. Good luck!