Why do you feel like you NEED a 4th A level?
If you are dead set on wanting a 4th A level, and if you want a science-related degree at any of the universities you've mentioned, further maths will be more helpful - far more so than RS, it is a much more respected A level by universities. RS won't set you apart from equally talented candidates, but further maths will. I take further maths and its my favourite subject.
I take further maths as a 4th A level along with maths, physics, geography, and I will say FM takes up FAR more of my time than the others (and the others take up a big chunk anyway, especially physics).
To be honest, I'd tell you to forget the 4 a level thing and stick with the 3 you've got - further maths isn't a requirement to get into biomedical or biochem, its not even on the recommended subjects anywhere for Oxford. Take 3, get top grades, and do LOADS of wider reading, e.g. reading New Scientist. Get some work experience etc., you'd be better spending your time doing that.