@ Antonio Correia. Sorry for the long response, but I know you like details and facts. Hope you can understand all.
As you have found out,
ALL hard drives will eventually fail. HGST (Hitachi) was the most reliable for years, and most Western Digital and Seagate HDD's were very poor in ratings compared to them, Like 6 to 10 times more likely to fail quicker. See the Backblaze Data Center chart below from 2017 showing 2014-2016 hard disk drive reliability from their BackBlaze data farms where they have thousands of these drives. I bought a USB 3.1 HDD RAID enclosure back in 2014 and put 2 HGST 4TB Deskstar drives in it, for a 8TB RAID, I also had 4x 4TB Deskstar drives in the internal bays of my 2012 Mac Pro and RAID'd up 2 pairs, so I had 2 internal and 1 external 8TB RAID setups with the 6 HGST drives. They never failed or had any problems, they were rock solid for almost 6 years.
Western Digital bought HGST back in 2018 and quietly closed them down by late 2019 as this level of quality was too expensive for them to make. But they did learn from HGST and are making better enterprise class drives as a result. Last year I upgraded to 12TB RAID and as HGST drives were no longer available, I bought Toshiba Enterprise class HDD with a 256 MB cache and a MTTF (Mean Time To Failure) of 1.4 Million Hours (The old HGST were 1.7 Million Hours.) They are housed in a Mobius Pro 2 bay USB-C enclosure hooked up to my 2019 iMac via a USB-C connector. Below is my typical Disk Speed Test R/W speeds via this setup I ran today, there is currently 8.1TB of data on the 12TB RAID. This is about 2-1/2 times faster than a desktop single HDD via USB-C or USB 3.1 (WD My Book).
While G-Tech, LaCie and many others quote very high transfers speeds in their literature via Thunderbolt 3 at 40GB/s, or USB-C at 10GB/s or USB 3.1 at 5GB/s, you will NEVER get anywhere near these speeds when you have spinning platter HDD's in the RAID box. The absolute best you can expect, even via TB3, when brand new and no data on the hard drive is a R/W speed of near USB II speeds, about 400 MB/s, and that is what mine tested too when I put the last RAID box together. But as you add data it slows down the transfer speeds, as the HDD need to read farther into the circular data path to get data the fuller it gets.
This means your don't need a Thunderbolt 3 RAID box that is up to $300 dollar more than a USB-C or a USB 3.1 box, the hard disk drives will not spin any faster just because the cable thruput is faster, the hard disk drives are the slowest part of the system.
What I would look for in the HDD RAID is make sure the MTTF is at least 1 Million Hours, have at minimum of a 128 MB cache, run at 7,200 RPM and are "Enterprise Class" with at least a 3 year warranty. These hard disk drives will cost $30 or more than lesser quality same size drives, but they will last a lot longer. You said you want only 4TB total, but I would double that as the bigger the RAID and the more empty space it has, the faster the R/W speed will be. High quality 4TB hard disk drive are pretty cheap and having a 8TB total RAID will save you going thru this a couple years down the line. My Mobis RAID box cost $130 and the 2x 4TB Western Digital Gold Enterprise drives (highly rated, with a 5 year warranty) can be had for about $149 each, so total of $430 for an excellent 8TB RAID system. Yes you have to slide the 2 Hard disk drives into slots in the RAID box, but it is so simple and a 2 minute procedure.
My Lightroom catalog is about 180,000 photos and kept on the Mobis RAID, and the cache file is near 250GB, yet it runs just fine. I do keep the cache file on my 512GB SSD internal blade in my iMac and this makes the catalog load a LOT faster. For backup I would buy 2x 8TB WD desktop USB 3.1 drives (My Book at $160 each) and back up the RAID to both of them weekly, if not daily, depending upon your activity. Keep 1 for sure in a fire safe.
Lower % is Better
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My 12TB RAID Mobus box R/W speeds via USB-C to 2019 27" iMac
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