Great question - we are indeed about to release a 4th edition!
That doesn't mean you have to wait, though - most of the changes are relatively minor, so if you already have the 3rd edition books, you don't need to rush out and get the 4th. We've already published an analysis of the changes on our web site, free to all, and we've also published an updated problem list for OG12 (so that you can do the problem sets out of OG12 even if you only have the 3rd edition, which lists the problem set numbers according to OG11). Here's the link to that area of our web site:
https://www.manhattangmat.com/official-guide-12.cfm
I believe the 4th edition is going to be out towards the end of this month, so if you're not planning to take the test very soon and can wait a couple of weeks, you might as well! It will reflect some of the changes in emphasis that we've seen from the recent official publications - but there's nothing totally new on the test that did not exist before or anything like that.
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Stacey Koprince
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Manhattan GMAT
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