We actually have a page dedicated to the differences between our fourth and third editions, which you can see here: http://www.manhattangmat.com/fourth-ed-info.cfm
Two key differences are:
1. the reference to the new OG (12th edition) in our fourth edition
2. the split between basic and advanced content in our fourth edition
If you do buy the third edition and you're using the 12th ed OG, you'll need to download the conversion documents on that page.
We always recommend that people buy the latest versions of our books if possible, because we do make content updates with each revision, but you could probably do all right with our third editions.
The only other thing to keep in mind is that if you're buying them used, you won't have access to our online CAT exams and question banks (which usually come with our books) as that 1-year access would have been registered with the original owner of the books.
Let me know if you have any other questions!
Student Services AssociateManhattan GMAT
Caitlin Clay | Manhattan GMAT Student Services Associate | New York
Manhattan GMAT Discount | Manhattan GMAT Reviews