There’s no Roon Server build for the ARM CPU architecture used by Raspberry Pi boards. Even if there was a build, the processor in the RPi4 does not meet the minimum specs.
That said, there are a few boards with a similar form-factor that do use an Intel CPU architecture that, in theory, should be able to run Room Server, even though they too are probably below the minimum recommended spec. For example:
Could actually be an interesting project. Just go in with low expectations. A board with eMMC or (ideally) M.2 NVMe storage is a must. Boards that only support microSSD will not have enough disk I/O performance to support Roon’s database.