Yes, 4K sounds a bit high, esp for someone who is trying to keep GMAT Prep Software Tests for practice and can't unlock/access those questions. I am not sure how you get 1,500 for free, however.
In any case, you have the OG with 900 and Verbal/Quant OG's with another 600 together, so that's 1,500... that you have to buy a book for. There are also some paper and older OG's and Question Packs and Exam Packs but they also cost money and you want to save GMAT Preps and Exam Packs (there are ways to find/extract all questions out of the GMAT Prep Software and there are a lot of questions, but again, they can't be unlocked until you exhaust those tests, which may be too late). In addition, most official questions outside of the Official Guide will not have an official explanation. Moreover, most of them will be medium to low difficulty.