Raspberry Pi 4 as a low-end server (eg. NAS)

Beside multi-core, Pi 4 spec also includes Gigabit network interface and dual USB 3 ports, that makes it a good platform for a NAS with RAID 1 disks setup  ie. mirrored, duplicated

A 2GB RAM ($ 35) is good enough for this purpose.  However, the more RAM the better in my opinion - also my recommendation is to download the Raspi OS 64-bit lite version (no GUI).  2GB is too weak for GUI; you would soon experience lagging in responses; the system will write frequently to swap - the SD card, reducing its life span.  Skipping GUI saves you ~100 MB RAM.  With more memory, other servers and/or even docker containers can run.  ;)

A very straight-forward instruction is already available by mag pi.

https://magpi.raspberrypi.com/articles/build-a-raspberry-pi-nas