Definitely don't postpone using the Manhattan study guides. Much of the study guides will probably be the exact same. For instance, if you are learning Sentence Correction, concepts such as essential clauses vs nonessential clauses, subject-verb agreement, and the semi-colons are still the exact same (and MGMAT has always been doing an excellent job of this). Triangles properties followed by excellent practice questions probably can't be much improved upon. I'm guessing the main changes will reflect the new Integrated Reasoning questions, and maybe--though this a stretch--one of the new 150-ish questions will have some unique twist that none of the MGMAT guide account for. Otherwise, the GMAT is more or less the same, and the 4th edition MGMAT guides
will continue to provide nonpareil prep for the MGMAT.
Of course, if it turns out that there are some excellent new tips/practice questions in one of the new guides, you can just purchase that guide. In the meantime, you will have learned all the fundamentals and valuable tips from the 4th edition.
Best of luck
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