Hi, I took five AS Levels; Maths, Further Maths, Physics, Chemistry and Spanish and I am now going to uni to study Chemical Engineering. I decided I wanted to drop one because I would have prefered to get better grades in 4 subjects rather than average grades in 5. Also, university offers are (generally) for 3 subjects, so taking 5 is excessive! I decided to drop Spanish even though I really really enjoyed it because essay writing took up a lot of my time so without it I could really focus on my other subjects. Although I missed Spanish lessons I still keep up an interest in Spanish at home so I don't feel I'm missing out. I felt I made completely the right decision - I found that there is a jump from AS Level to A Level difficulty wise, especially in Chemistry and Further Maths, so dropping a subject really helped me to keep on top of them, and I don't think I would have been able to achieve the grades I got in my A Levels without dropping Spanish. I would definitely recommend dropping to 4 A Levels!
In your case, I would ring up/look at prospectuses for the arty courses you're interested in and see whether art is a recommended or necessary subject - you may find you don't need art at all, you don't need Biology to study Medicine! Chances are you may not need Art A Level and universities will be able to see that you've got an AS Level in Art so that puts you in a strong position.