Why the Pi4The Raspberry Pi in general is a great hardware platform for a quick DIY project. The Pi4 in particular offers a nice balance of performance and size. It's just small enough to fit anywhere and just powerful enough to run some beefy servives including some real time transcoding in Plex. There is also a huge community out there that supports the Pi both as hardwaree and software.
The SoftwareI always start with a fresh Raspbian installation. Regardless of the size of your microSD card, I recommend keeping a fairly small partition (I typically use 32GB) for the OS and partition the rest as an NTFS data volume. NTFS is decently supported under Linux these days, supports symbolic links, and can be mounted easily enough under Windows for quick access to files. If you do this, be sure to add a matching line to /etc/fstab so it is automatically mounted on boot. On the data volume I will typically have my on the go collection of music, video, ebooks, and other files I'm using.