3 or 4 Alevels?

There is no real reason to do 4 rather than 3 A levels unless you have 4 subjects you really love and are unable to choose between them as all university offers will be based on 3 A levels. Having 4 A levels may make you look that bit more impressive to universities but only if you can achieve extremely high grades in all of them (for example, it would be preferred to have three A*s instead of 4 As) so it's a bit of gamble. There are also other ways you can make yourself more appealing to unis such as extra curriculars that really demonstrate your love for the subject (doing an EPQ would be useful for that), you would probably be a better candidate if you has 3 A levels and extra curriculars that showed your passion for the subject than if you had 4 A levels but had done less extra curriculars to make time for them.