Literally, I took 4 A levels last year and came out with U,E,E,D. It was a complete waste and I actually tried so hard. I highly recommend taking 3 because you can focus your revision and if you want to go to uni, they only look for 3 Grades...
I had to fast track politics this year so technically have 4 subjects again and I've struggled just as much. Also, by taking 3 you will have an extra free slot in your timetable to keep up with homework, coursework etc!
There's no need to push yourself harder than you need to and you have a better chance at getting higher grades with less subjects. Im at the 'best college/sixth form' in the UK and they don't actually let anyone take 4 subjects unless they're 100% capable of getting 4XB's or above..