Some ramblings on the subjects you're talking about:
In a Data Center environment, I can't recall the last time I saw a problem with a HARDWARE RAID controller. IBM, HP, even Dell have been very solid in my experience. Really massive storage tends to be in large disk arrays (30 to 150 drives) that are in RAID sets and get carved up and presented to hosts for their use. I can't recall EVER having a failure in one of these large boxes that resulted in data loss. In both big and small RAID boxes, I've seen disk failures. But RAID took care of the rebuilds and all the data was safe.
Not to say that it doesn't happen! I'm just saying, in my professional career with computers, RAID failures don't seem to be that common. Maybe I'm just lucky or have only had exposure to really nice RAID units.
On the other hand, RAID is not a backup. So you're failing from the start if you're not incorporating backups of the data that you care about. You have 3+1 drives in each enclosure (RAID5), which implies up to 12 TB of data in each enclosure. Times 4 enclosures, that means about 48 TB of data (video I think) that you probably care about. For me, job #1 is to get a rolling backup of this data going. Otherwise, when you lose a whole enclosure due to your expected total RAID failure, you lose 12 TB of data that you can't get back without serious effort.
Your idea of putting them all into a large PC type case raises huge red flags for me. Heat is the enemy of drives and that many drives needs forced air cooling. I wouldn't try it myself. Being a DIY guy is fun and all. You can save money and claim "I did this all myself!". But what happens when the whole thing overheats because you aren't experienced enough to know what the proper temperature is, and you lose a bunch of data?
Personally, I would be looking into a plan for where my data is going in the next couple of years. Is it growing? If so, how much do you expect to have in 1 to 2 years? Plan for that. If it's pretty much done, then figure out how to get your data into reliable enclosures that won't make you wonder if your data is safe. At the same time, budget or purchase backup drives. Backup drives don't need to have RAID protection; they're just backups.
The good news is, drive sizes continue to grow. 8 TB drives are twice as big as 4 TB, so you only need half of your current drives for the same data storage. Which reduces your likely hood of drive failure. The more drives you have, the more often you can expect one to fail.
Brian.