Taking more than necessary does not necessarily mean it's better. Taking 3 may mean more time to look outside the syllabus to broaden your knowledge before you go to uni.
I took 4, but physics chemistry maths and further maths, to do engingeering.
Was offered AAAB (B in chemistry). i think like the letter said its not vital that i know A-level chemistry to do engineering, but they like to see it, hense they offered the B.
I have a friend with an offer to study physical nat sci at Queens, he took 5 A2, I think in maths, further maths, physics, chemistry and german. He was offered AAA in FURTHER maths, physics and one other (NOT MATHS). We don't sit the whole of maths A2 in year 12, we have 5 maths and 1 F.maths module.
I would think that if you're applying for a science or engineering or maths, and your 4th A2 is F.maths or an extra science etc, then go for it, if you are a linguist or an arts sudent don't bother and read more stuff that you are interested in!!!
In short - no it doesn't matter. Just as they're looking to see that at Oxford you spend your time wisely, know your limits and don't spread yourself too thin- clubbing, or rowing or drama for example; they want to see you commit to your exams properly. 3 a's stands up better than 3 a's and a b. Or, of course, you can look at it technically - they can only expect you to get 3 a's and therefore cannot penalise you, although applying with 4 a levels will ENSURE that they offer you 4 a offer, so you'll have to do even more work. At the end of the day what the tutors say is right - almost every candidate that applies to oxford is intelligent enough to study there. That's not what they're worried about. They just want you to be sparkling and interesting at the interview so they don't have to teach chronically boring people.
I agree that most offers are only for 3 A levels, even for those taking four or more. Occasionally they ask for an A in the fourth or they may be specific about which subjects so you may find them asking for an A in your least favourite.
wadham do seem to give out a lot of 4 alevel offers, it seems to vary with college.
...but rather than being lots of AAAA offers, they've given out lots of AAAB ones.
Now in that fourth subject I was not going to get a C, so really all these offers do are to make sure you continue all four subjects as you said you would, which is fair enough really.
i did 6 and got an offer for 3 with no conditions on the other 3. i dropped one, and erm, buggered up another 2. so its quality over quantity, provided you do do 3.
When i heard a talk from a Cambridge Admissions Tutor i heard the best explanation of an answer to this questions - he said "Three A-levels in themselves are both sufficient and not sufficient". Thoug obviously this is contradictary it is remarkably true. If you want to apply three A-levels is fine (3 A's of course would be expected) however it is not sufficient if that is all you have. You need to demonstrate a desire to learn, an excitement about your subject. Doing a fouth A-Level is a way of doing thism though it is not the only. Going out getting work experience, debating, sports, etc. are all ways of doing this.
So to finish- yes it is fine to do 3 A-Levels though it can't be the only cards that you hold in your hand.