What should we do in the last 40 days of the NEET preparation if I score around 400?

You should revise, get to know what are your weak points. Work on that. Revise NCERT books again and again. Revise what you already know. You can also take help of basidia learning app. Follow their keynotes, save difficult and important keynotes. Revise them. They have 2000 keynotes. Which covering full NEET Syllabus. You should take the Mock test and Practice test as many as possible. Students who are interested to take NEET21, you can start taking practise test now from that app. Their MCQs are very convenient.