Availabiliy is still bad I'd say ... at least ovre hee in Europe. However, I'm always open for freely available disks
We have seen the first reports of Seagte 4 TB drives pop-up in the forum. Logically and tecnically, there should be nothing that stops you from using the 5K4000 drives. Following the history of qualifying "supported" drives, I tend to say a good level of trust can be established over time only. Passing a simple test should be easy, however nobody can say what will happen when the disks start to create issues, ie. blockc replacements, error recovery, ...whatever. From this view, the value of a "supported" drive listed is limited.
When you have prfromance concerns, and one of the faster QNAP Ingtel-based NAS, I suggest to go for 7200 RPM drives.