Last I looked, the best price per TB is for buying external drives and pulling them from the cases. Might not be the case at 4 TB though. One thing that trips people up is sata power wire pinouts have changed over time; 3.3v power was removed and one of the 3.3v power pins was repurposed as a signal to inhibit spinup; if you have a power supply (or other sata power connector) that supplies 3.3v and you get a drive that uses it to inhibit spinup, it won't show up. The easiest fix is to clip the orange wire, but some people use kapton tape over the pin on the drive.
At 4 TB you also have to worry about drive-managed SMR drives, SMR has some benefits in some use cases, but they go out the window IMHO if the host doesn't know about it. This is true for bare drives and external drives, but external drives are harder to know the real specs for.
