Littlehum, even amongst DD's/my friends there are huge differences between approaches.
DD's school - every subject this year is an external AS. Those who are doing 4 drop one to continue with 3 to A'level.
Her old school - mix of "old" AS's and "new" internal exams. You can drop one, but if it's not an old AS, you won't get a qualification.
Another local comp - they only take 3 A'levels and only do AS's in the "old" subjects, internals in the new.
DD has no idea which to drop - at least it will be worth an AS and not a "wasted" year, otherwise you might as well only take 3 if they are "new" subjects