I want cheap 4TB SSD (no matter the connector/format) to start to build a NAS on RAID 5. With Synology's hybrid RAID I could buy a single 4 TB SSD and then gradually buy more drives with equal or bigger size as they get cheaper (I'm going Synology almost exclusively because of its hybrid RAID, which no other brand seems to have a built-in equivalent yet). My goal is silence and longevity (as in how long the drive will last before it breaks), which SSD drives seem to top HDD by far. I'd rely on btrfs to guarantee data longevity. Hopefully this Phison drive will have competitive prices, but I doubt the 4 TB one will go down in price enough to be worth it.

Memory density is going up, and although I wouldn't buy QLC for everyday usage, it's an amazing solution for mass storage. I'm in need of 4Tb of storage and considering an HD, I wish I could afford these right now...

There's nothing wrong with using QLC in everyday use. They wouldn't put the same warranty on them as TLC and MLC if they weren't designed to last as long; they'd lose their shirts if they didn't. The real drawback is speed, but when you are bottlenecked by the SATA bus anyway that really doesn't matter. I am about a year and a half of use into my 4TB 860 QVO, with almost everything on my PC running from it, and have not seen one hiccup. On the other hand, my old 240GB Micron MLC SSD (now is the boot drive on my server), has been warranty replaced twice in the 4 years I have owned it, and I noticed it getting SMART errors again over the weekend. It's going in the bin when it dies this time, and getting replaced with a cheap M.2 drive - I need the SATA port for another big storage HDD drive anyway.