If they are saying there is a 4tb limit, that might have something to do with sas backplane of the ds4246. If it is older, then this might be a real concern that requires you to buy a new backplane.
Good ole’ fashioned redundancy. You probably don’t need it for what you are doing. You would need an hba card that supports that connector.
The right hba and cables depends on what you are trying to connect your drives to. Whether it be directly to the drives, to a backplane with individual drive connectors, or to a backplane with one or two cables. How you decide you want to approach it depends entirely on what you want and how much you want to spend. I couldn’t tell you about what is in the ds4246.
You have a problem though, that second slot is only a pcie x4, so you can get a maximum of 8 additional drives without losing any speed. You can get a card that goes over that limit, but then you are limited by bandwidth. To get more drives at full speed, you will need to remove the gpu or use the 1x slot for another drive or two.
If you want my personal recommendation, unless you already have a server rack or plan to need one for other servers, just get a large pc case like the fractal define 7 xl, upgrade your power supply, and buy an hba card and cables that allow you to connect to as many drives as you need individually. Why would I recommend this? Because enterprise servers like the ds4246 are loud as fuck. You don’t really realize how much sound matters until you live with a jet engine. That and because adding a jbod is adding more complexity than you need for your build. More parts to fail. You could probably get your required parts off ebay for cheaper if you’d like.
There is also the question of whether or not your board supports hba cards at all, so keep that in mind for your research.
If you need more drives than that eventually, you are going to need to upgrade to a motherboard that supports your needs. At that point would it be worth looking into a rack mount for more than the 18 drives the fractal case can handle.
If you want more information, I need to know if you plan to use my idea or not. As of right now, you can’t properly use the ds4246 chassis without removing the gpu and using that pcie slot. Or use a card that allows you to but limits your total performance.
If you don’t mind me asking, what is the main purpose of this server? If you need the gpu, I assume you are using it for plex or something.