Hi there. You're clearly a smart person who has made the disorganised (but save-able) decision to leave revision until the last minute. Take a deep breath, all will be well. Most of us have been exactly where you now stand.
Remember this: time means nothing. Motivation, and good planning, is everything. If you plan your revision well, you'll be able to get more done in four weeks than most people could achieve in four months.
So what does good planning involve?
1. Set goals for the topics and subjects you'll cover each day. Stick to them.
2. Invest in some good revision guides, especially for the subjects you had bad teachers in. Actually, learning to teach yourself topics can be one of the best skills you'll gain, that will stand you in good stead for university.
3. Watch, as some other commenters have recommended, some YouTube video series, but - here's the trick - do not waste time. Only watch a video if it is relevant to a topic you are studying, that you find difficult, and need a bit of extra explaining to understand.
4. Know thy syllabus! I cannot emphasize this enough. You'll find copies of subject syllabuses online, on the websites of the specific exam boards. Memorise the checkpoints of what you are expected to know and then go learn them.
5. Past papers and questions: do them. You will only realise how little you know - and be able to correct it - if you prioritise answering these. At least one a night. Do these under exam conditions, try to figure out how the mark scheme awards points, and learn to write like this. Many intelligent people lose marks because they fail to understand what examiners want to hear. So, while you may feel you have no time, make the time - this is an investment that will definitely be worth it.
Good luck! If you have any more questions, I'm only a message away. As someone who found themselves in the library, the lunchtime before one of their final IB Biology unit exams, trying to finish revision, just know that I completely emphasize with your poor plight, and am here to offer you hope.