Re doing 4 A Levels, bear in mind that for those applying to Cambridge it’s quite common to be given an offer based on all 4. So what would have been eg A, A, A for 3 becomes A, A, A, A for 4 (or even higher.) As far as I know, Oxford stick with their standard offer regardless of how many A Levels are being taken. At least that was the case when DD was applying a couple of years ago.
DD did 4 because she didn’t want to drop any of her subjects. She didn’t do an EPQ because a) the way it’s structured didn’t really appeal and b) she was doing Pre-U English which, as Kingscote says, involves writing a mini-dissertation on a subject of your choice. She also entered, and won, an Oxbridge essay competition which took up quite a lot of time. Plus there is the preparation required for the Oxbridge entrance exams, and the interview, in the first term of Y13. Added to the workload of 4 A Levels, she was stretched to her limit in that term particularly.
For potential Oxbridge applicants in particular it’s just worth bearing in mind that they won’t ‘only’ be dealing with the A Level workload.