Fedora doesn’t currently support the Pi4, although it appears people may have been able to get it working. I would personally just not risk it and go with one of the supported operating systems, but there is probably no easy way to tell unless you buy and test that hardware.
I am running Fedora IoT 33 on my RPi4 atm. And the arm-image-installer package does list the rpi4 as a supported target ! (That’s how I installed it, from my Silverblue desktop.) Maybe that page needs to be updated? Or maybe support hasn’t been tested enough to be considered official?
I would personally just not risk it and go with one of the [officially] supported operating systems, but there is probably no easy way to tell unless you buy and test that hardware.
The board is attached via usb so it should work right away. Booting the pi4 from usb requires a recent firmware/bootloader. I’m using F33 Xfce with an ArgonOne:M2 & ssd as a daily driver without problems. Basic functionality works out of the box, sound and graphics might need a little tweaking but should improve with kernels 5.10 and up.
Sure there are no optimisations for the pi4 (yet), but everything required for a (headless) server should work.