I assume you are talking spinning rust rather than SSD's? I suppose "cheap" is relative, but I think I can speak for most members when I say that 4TB SSD's generally do not fall in the "cheap" category.. I have a large qty of 1.92TB SAS3 SSD's listed here in the for sale section that are probably "cheap" relative to other similarly spec'd SAS3 SSD's..
Other than that, I have a single 1.8TB SAS3 2.5" 10K RPM HGST drive I'd be willing to let go of, but this obviously does not meet the requirements stated above..
Are 4TB non flash 2.5" SAS3 drives a common item?