My sixth form is making a lot of people drop subjects now, so it's certainly doable. I also think you won't be too disadvantaged when applying with only 3 AS levels due to the new reforms. I'd say go for it if you really don't think you'll end up with 4 As.
If the OP withdraws from the exam officially the she won't get an X. Your school may make you pay for the exam fee entry since they won't get a refund.
That will be down to her and how well organised the school is.
Even if she withdraws successfully, she will be at a disadvantage. It's just like not taking FM when you're applying for a top maths degree. If it's on offer to you and you don't do it - they will question it.
I dropped Psychology before Easter and there were no problems..I think it depends on the school and if they're willing to let you. Personally if you're aiming for Oxbridge then I wouldn't drop it..but other Russel Group unis like UCL wouldn't mind.