As posted above by hsalika, Oxford Maths and Computer Science offers are based on three A-levels, including Maths and preferably Further Maths; it would be very unusual for a fourth A-level to be included in the offer. If you want to and feel you are capable of doing more than three A-levels without risking making the standard offer grades then by all means do so, but we'd rather see you focus on three A-levels and do really well in them than do four and perhaps miss a grade.
We strongly recommend Further Maths, and the vast majority of successful A-level applicants to Oxford Maths and/or Computer Science courses have done Further Maths, so I wouldn't recommend dropping that if you want to apply to Oxford Maths and Computer Science. We would certainly view Maths and Further Maths as two separate A-levels, not one. However, other universities' views on this may vary, so you should check with other institutions you're interested in applying to, to check what their view is on 4 vs 3 A-levels and whether they count Maths and Further Maths separately or together.
Rosie
Department of Computer Science, University of Oxford