You dont need 4 unless you are truly interested in that 4th subject and its so much advantageous for you to do 3 instead. For law, you can choose whatever and every subject will help you develop the skills for law. If you want to keep your options open, focus on the other careers you want to go to so if you would go into environmental sciences, research what you subject it requires as different unis want different subjects. You might need between Chemistry, biology, geography, maths, physics or psychology but i cant remember fully but I assume you need at least one science.