I'm currently looking at one of the Synology NASes just for simplicity, and the system will be next to an air purifier I have running 24/7 so moderate noise isn't a huge concern.
Any recommendations?
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.
I mod the OS to turn off certain options between 10pm and 7am so the HDD can sleep during night hours.
HGST has always had the best reputation for longevity, but the others have caught up a fair bit in recent years.