4U scales great and can bring up your aggregate, but won't help you more than most other subjects if you're not good at it. In saying that, if you work super hard I'm sure you'd be able to succeed if you're a medicine-hopeful, but don't feel pressured into doing subjects you don't enjoy/aren't good at because of stigmas about medicine. You don't need it, but it may help show you are interested and committed to science and maths based subjects.