It depends on your school if they allow you to sit the AS exams, if they do you can drop the subject after a year and have the AS qualification. I did biology, chemistry, maths and spanish this year and I'm dropping spanish next year (1 week time) , but I still have the AS qualification. If you want to do 4, you can do it if your school allows you. If not there is no point to do one year and drop it as you won't have any qualification.
Hope that helps. Good Luck!