Bored of SC, I think that being honest with yourself is important. Are you really understanding the math in 2/3 unit math at the moment at school? I reckon that 4 unit math requires persistence and hard work, but even then, you have to understand what's going on, as opposed to rote learning, which you would be able to get away with in 2/3 unit maths.
Mathematics, like any other science, is something you need a passion for. Sure, the BOS wants you to be mechanical, good uai-churning students and good marks really do count, but if you enjoy your subjects, and have a willingness to learn, 4 unit math should be an 'inviting' challenge.