RS as a fourth A Level is unlikely to help you (it won't hinder you, but won't make your application stronger either) in Medicine applications. You are better of doing three subjects well than four slightly less well. Bio, chem and maths will keep all medical schools open to you, as well as other options if you choose not to take the medicine route. By all means take RS if you think you will do well in it, but not at the expense of the other three. Good luck!