How much size do you want?
Do you want protection against hardware failure?
IMHO, and while I am anything
but an expert on the subject...If you want pure economy without redundancy, you could go for the Mobius 2-bay drive and two 4 TB WD Reds in a RAID 0, giving you 8 TB of storage - but, should one drive fail, you either have to bite the bullet for several hundred-to-several thousand dollar data recovery, or accept that all of the data on both drives will be lost. Alternatively, you could use the same products in a RAID 1 configuration, giving you 4 TB of storage, and redundancy in that each drive would automatically store the same files as the other (arguably much more desirable if you wish to protect against data loss from hardware failure [but not corruption].) Also alternatively, you could use the drives in the 2-bay enclosure to manually back up the files yourself to maintain one version of your most important files on each drive. You could do any of these options for around $400.
If you wanted more reliability, a longer warranty, and/or longer anticipated service lives, the HGST Ultra Star is my biased choice, but it would be pricier (about $100 more per 4TB drive over the WD Red.) I absolutely love these drives and swear by their reliability, ability to withstand 24-7 usage, and long service lives - so I am biased here. Like many other 7200 RPM 3.5-inch enterprise drives, they are extremely fast for HDDs (in many cases, over 200 MB/s read/write) and rather loud.
If you wanted expandability, one of the 5-bay enclosures might be ideal. Even if you do not use RAID at all (or use soft 1/0) for now, the 5-bay HW RAID enclosure would give you the ability to implement hardware RAID 3/5/10 in the future, should you desire, and the independent 5-bay means you could implement a software RAID 1/0 and use another disk independently to store the same files using a different means (so you can account for both hardware failure
and software corruption.) Especially if you plan to continue to accumulate 4k video, a 5-bay enclosure gives you a lot of expandability where you could use your existing enclosure and just add additional drives to it (versus having to purchase another enclosure.)
Possible Enclosures?
USB/eSATA/FW 2-bay, and capable of hardware RAID0/RAID1
USB 2-bay, and capable of hardware RAID0/RAID1 (this is a relatively new budget line product and I imagine it is not built to the same quality as their pricier dual and quad bay enclosures?)
USB/eSATA/FW800 and capable of hardware RAID0/1/5/10
Thunderbolt 2 & 5-bay, and capable of soft RAID
Possible Drives?
6TB Desk Star
4TB WD Red
2 TB Ultra Star
4 TB Ultra Star