If you're doing 4 A-levels then it's not an issue, as you'd still be doing 3 in one go.
Basically for Oxbridge and some similar unis, and for courses like medicine, they often prefer or require you to have done a "full academic courseload" during your A-levels, including doing a full set of exams in one sitting. For Oxbridge this is due to how their courses work (largely assessed by final exam and so they want to know you can take multiple exams in one go successfully) I gather.
Honestly if you're doing 4 A-levels I'd say not bother taking it at all and just dropping it. You also don't get "bonus points" for doing a 4th A-level as far as UK unis are concerned so just dropping down to the normal required 3 A-levels would be fine and probably the best approach.