1. Copy and paste the syllabus into a Word document and put each bullet point on a separate page
2. Break each bullet point down into a few essential things that you need to know
3. Focus on each essential thing one by one. Work out how many hours you could reasonably do (I would recommend not doing more than 7 a day because you need to give yourself breaks otherwise spending that much time will be useless) and give yourself a limit for how much time you should spend on each point.
4. Tick each point off as you go, to prove to yourself that you are progressing.
4 days is a long time and if you are this worried then I am sure you are motivated enough to have already revised a lot more than most people. It may not feel like you know much but when you get into the exam you will be able to recall so much more than you think and will probably remember things from lessons that you haven't even revised.
Most importantly, though, think rationally about this. I don't know what year you're in but if you're at GCSE then don't worry at all, GCSEs basically mean nothing haha. If it's AS, then unless you drop the subject you will re-do it in Year 13 anyway and universities on the whole are more concerned about predicted grades than AS grades, and if you're at A2 then this exam will be worth less than about 30% of your overall grade - you have your AS and the other exam/coursework unit to fall back on if this exam doesn't go as you hoped.
The best thing you can do for yourself is be optimistic and think rationally. I know it is hard and I have been there, but this exam means little if you think about the bigger picture.
Good luck, I hope this helps even a bit