NEXT-LEVEL NAS
If you’d like slightly more complexity to your NAS than a simple Samba share, look in the direction of OpenMediaVault, and cross your fingers that it gets updated to the Pi 4 soon. It’s a much more complex and fully-featured NAS solution, with a web interface that lets you do the dirty work of setting it up and allows you to access files on your server from outside your local network, if you set it up correctly. It also offers access to services like SSH and FTP, allows you to run torrents on your NAS box, and much more.
While there’s no official build for the Pi 4 yet, you can download an entire single-purpose OMV distro for its elder siblings at https://sourceforge.
net/projects/openmediavault, which you just need to write to a card with Etcher—as with all these not-officially-compatible projects, you’re welcome to try it to see whether the current 32-bit version works for you. OMV’s distro is based on Debian, which has been proven to at least vaguely work with the Pi 4, so you should be able to run it without too many hardware conflicts.
Run through the installation when connected to a mouse, keyboard, and monitor, logging in with the default username “root” and password “openmediavault.” Use the command ifconfig to determine your Pi’s network address, and head to the OMV web interface from another machine on the same network by dropping that location into a browser address bar. Logging in as “admin” with the same password should get you in, and you can start configuring straight away. No guarantees, though: A few months’ more Pi 4 maturity should get you a version directed at the new board.