If it was me, I would stick with the August 15th date. I know how it seems like time is just flying by and there's not enough hours in the day to prepare, but if you put in the work during this next month I think you can rock it come test day. A 27 on AAMC 3 is a great starting point! I only got a 22 when I took it, you're already in a better position than I am. AAMC 3 is an older, easier test, so your next score is likely to stay the same, or only fluctuate a couple points. That's okay. Just expect that going into it so you won't feel super discouraged if that's what happens with AAMC 4. Then for these next couple of weeks leading up to test date, take as many of the AAMC exams as you can, but allow yourself sufficient time in between them to review the entire exam thoroughly and not get too burnt out. I am taking one a week, but I started taking FL exams earlier this summer so that I could fit all of them in. Just do what you can.
You can do this! There is SO much content for the MCAT, its simply just unrealistic (for me at least) to be able to know everything 100%. However, we all for the most part have topics that we prefer, that when we get a passage on it, we feel comfortable and can answer almost all the questions correctly. Know THOSE topics 150%. For the topics that you are just really struggling on, obviously spend time reviewing it and learning it as best you can, but be okay with knowing it maybe only 75%. That's how I am approaching things.
Keep working hard, put in the time every day (maybe take a day off here and there), and you'll get there. At this point, now that you've gotten through all the content, I think your most valuable tool will be taking practice tests and any other practice passages/questions of course, but more importantly reviewing them. Review goes for any FL, subject tests, topical tests, etc., any kind of practice passages. Review, review, review. This is where you will see what content you're missing and can then plan your content review accordingly. I have a "content review" day built into my study schedule now. Just once a week, but you could do more. I make a note of a specific topic from bio, physics, gen chem, and orgo that I feel like I need to cover again, and that's my study day on Sundays. If applicable, I make study sheets, and then make copies of it with different parts blanked out.
Having said all of that, if August 15th feels so close to the point of it making you so anxious you can't really focus on studying, then push it back. $100 is worth the peace of mind to be able to focus on studying and get a good score. Just keep in mind that means your scores will be back even later (if you're applying for 2015).
Hope that helps. Hang in there!