I personally believe that there should not be a lot of difference. Each of the practice exam is as close to the real one as any other. I bought only 3 & 4 and I got 760 and 780 respectively in those and in the real one that I took 2 days after test 4, I got 770. Even test 1 and 2 were close to the actual score.
Also, I feel buying all 4 will be a better bet than buying advanced questions. You can always come to gmatclub and filter 700 level questions to practice harder questions. Having more tests to prepare you to sit in the real exam like conditions will help you better.
One more thing, just in case, if you feel you're not confident enough, and you might need to retake the test after your first attempt, really hoping that it will not be the case, you should try to save 2 official tests so that you have something to practice that is close to the real thing after the first attempt. Might help you get your confidence up. (This is absolutely my personal belief and undoubtedly is highly debatable, just putting it out if it helps you in any case)