I've never seen the point of people doing 4+ AS levels or doing any more than 3 A levels. By doing extra work you are risking dropping grades in your other subjects. A*AB is better than AABB, and you gain no advantage for doing more than 3 A levels in total.
But at the end of the day it is up to you, and if you believe you can handle it, then go for it. You could always drop down to 4 AS levels if you find throughout the year the work load is too much. I cannot stress enough though how important it is to keep on top of your work, and do not under estimate A levels.