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
For a NAS, you're usually concerned with capacity first. And you can't buy a 20TB m2.
M2 isn't much more expensive than HDDs anymore. It used to be five times times the cost. Now maybe 1.5 to 2 times. And they require a lot less space and no dedicated backplane with seperate power.
Also you need redundancy. A 20TB hdd is very expensive and usually very overkill. Rebuilding them takes very long if you ever have corruption issues, along with other problems.
It's better to have three smaller drives than a single 20TB one.
Unless you're running an actual datacenter with cooling and big rack space, m.2 is a likely more worth it these days for a small home NAS.