Four subjects is a demanding programme. You will need to take all four for the full two years, and will have a large number of exams at the end of the course, as well as lots of demanding homework all the time. Very few universities or employers will actually require you to have taken four subjects, and it is usually better to do really well in three rather than spreading yourself more thinly over four. The only areas where we would advise you to think of taking four subjects would be if you are considering applying for medicine, veterinary medicine, or for the most competitive universities like Oxford or Cambridge. Competitive universities may want to see Further Maths as part of this package if you are looking at a Maths, Physics or Engineering degree.