I can’t find a scan anywhere of anything 4a or higher, but if you are willing to pay for a physical copy there are a few sellers on AbeBooks selling “Reach Higher Student’s Book 4A”.
For free textbooks outside of that series:
BCcampus Open Publishing: They provide a ton of free courses and textbooks on a variety of topics. This isn’t really piracy, but it’s got a ton of good information.
I did find one site that seems to have all the PDFs for sale, but I can’t tell if it’s a scam or not. I would recommend looking at some of the free resources available on BCcampus before attempting this.