4 subjects won't give you an advantage in your ucas application; you're better off using the time you'd commit to a 4th a level doing things to demonstrate your interest in law instead. i'd stick to doing 3 subjects. your combination seems fine to me as long as they're all subjects you're interested in or enjoy.
i did a-level english lit and it's a lot of work but so are most a-level subjects. you need to be well organised in that you're reading the set texts at a good speed and doing plenty of practice essays.