Edit: thanks everyone for the suggestions. In the end I decided to buy a icy box usb3.1 4xhdd enclosure for around 100€. In the description it says it only works with mac and windows, but my Linux laptop works well with it, I guess the pi will to as well. I will print an enclosure for the power brick and the pi to screw to the drive case.
Here is why I choose this option: The pi is rather cheap compared to its power and also power consumption. I already had 4x4tb HDDs which I wanted to use, so at least 4 slots needed. The pi has 2x usb 3.0 connections which can be used simultaneously. One will be used for the 4bay, the other is free for now. I have the option to connect a second Nas case if I need more storage. Usb 3.0 is pretty fast, even if I don't get all the 5gbit/s. It's still faster than 1gbit/s ethernet.
I also thought about getting 2x 2xhdd enclosures to use the two usb3.0 at the same time. But decided against it because it would be a little more sketchy and I wanted to keep a free usb port for a second drive enclosure.
There are some enclosures that offer raid (hardware raid?) But I could not figure out if that would mean that all 4 drives will be raided, so I decided for the cheaper variant and would do the raid myself.
I plan on running 2 drives as raid1 and the other as raid 0 for secure storage and the other for movies and stuff I can download again.
Thanks again for all the comments!
It seems weirdly difficult to find a good solution to attach HDDs to my pi. Best case would be for me a enclosure with small power supply, space for my pi, and at least 2 bays for HDDs, rather 4. All that for under 100€ of cause :D
I could not really find cheap hhd enclosures that connect via usb. Any recommendations? I don't really want to use HDD toasters, they feel not permanent enough for a Nas. I could also not find sata to usb hats for the pi that are available right now
I did too, but shortly after decommissioning that server the drive became unresponsive. I really dodged a bullet without even realizing at the time. SMART data did not work and may have alerted me in that case.
Also, unrelated to SMART data, the server failed to do reboots because the USB-SATA adapter did not properly reset without a full power cycle (which did not happen with that mainboard's USB on reboots). It always git stuck searching for the drive. Restarting the server therefore meant shutting it down and calling someone to push the button for me - or use Wake-On-LAN which thankfully worked but was still a dodgy workaround.