4 days is quite a long time if you think about it, just divide up what you need to know for the exam into topics, maybe there's units or something that the specification is split up into. Then time table these across the 4 days you have and try and work through each one, focusing on the things you find you're weakest at, maybe making notes or flash cards, watching videos on YouTube. If you have time try and find some past paper questions specific to each topic and even if you don't do them it will be good to read them and get familiar with what the exam board might ask you in the exam. Just be calm, what ever will be will be ! Good luck !