I'm sorry that we're just getting to your post now. October-November is busy time on the forums and we get way more posts than we can answer in our usual timeframe of 3-5 business days.
I agree that it's disconcerting to have your score drop like that. You're going to have to dig into the data to try to figure out what happened.
Did you have any timing issues? (Note: "finished the test" isn't enough - you can have severe timing problems and still finish the test.) You can use this article to help analyze timing:
http://www.manhattangmat.com/blog/index ... ice-tests/In particular, look for what I call "up and down" timing - you spend a bunch of extra time on some questions and then you rush on others to catch up. This can lead to careless mistakes on the "rush" questions, or a string of wrong answers towards the end of the section, or something along those lines.
Did you do anything differently on that last test compared to the earlier 3? Did you NOT do essay and IR on the earlier ones and then do essay and IR on the most recent one? If so, then the bad news might be that the earlier scores were artificially inflated because you gave yourself a mental stamina advantage that you didn't get on the last test (and you won't get that advantage on the real test either). If this is the case, then the lower score might actually be closer to your "true" current scoring level.
Did you take the last test right after taking another practice test or after studying 10 hours a day for multiple days in a row or something else that would have *really* tired your brain out? Was there some other reason that you were really tired or distracted or burned out?
Look at the questions - did you make more careless mistakes than normal? Can you figure out why? The two most likely culprits are timing issues and mental stamina issues (often, a combination of the two).
If you can figure out why, then you can figure out what to do to prevent the same issue from occuring again in future.