Hi!
Ok so A-Levels are indeed becoming linear, meaning that you do all of the exams for the qualification at the end of the course (summer of year 13).
Because of the changes to A-Levels, most schools will only let you do 3 subjects anyway (unless you're a keen bean and want to do further maths as well) so it's unlikely you'll be able to do 4 but if you did, yes you could drop it at the end of year 12 (in most colleges).
aliyahh16 is talking about the EPQ (extended project qualification) which a lot of schools will be moving to instead of the 4th AS subject and it basically consists of a large research project and either an artefact or essay. It is worth the same as an AS in terms of UCAS points which will be 40% of an A-Level under the new UCAS tariff.