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.
You can buy a wd mybook and extract the drive, it’s cheaper than buying nas drives and if you get >= 6tb they should be equivalent to wd reds. You can checkout r/datahoarder over at Reddit, there’s much information on the topic.
I am happy Synology user since 2015. Currently I use Synology DiskStation DS918+. I fit in 4 x Toshiba N300 4TB 3,5". So far I'm happy with this mix. I use it mostly for backing up of my machines and my VPSes (over OpenVPN), and also as media server connected directly to my TV (I'm weirdo who is not too happy to have TV constantly connected to the internet).
I mod the OS to turn off certain options between 10pm and 7am so the HDD can sleep during night hours.