I have done some searches through the forums using a number of keywords, it seems there is little to no information regarding the "New" Raspberry PI 4 Model B Board.
In the compatibility matrix it states that the firmware is currently "snapshot" whatever this means with no download link or any way to try this "snapshot"
I am looking to test RDS Sessions using this device, should I look elsewhere for "Full Fat" Linux Distros that can perform this task?
Most likely a distro that includes xfce, KDE or whatever.
Regarding overall performance/usability it's probably worse than your average distro which targets the RPI4 as OpenWrt is pretty much mainline in that regard and most distros uses -O2 instead of -Os by default.