Hi Andrew,
A single Pi4 can perform about 14 receive channels of WSPR-2 decoding, so with two Pi4s you could support about 28 receive channels.
If, as I would hope, you want to add more than just WSPR-2 and FST4W-120 decoding, then you will need more Pi4s or a more powerful cpu.
At KFS I decode 28 receive channels all bands / all modes on a $400 Dell dual 12 core Xeon server which consumes 185 watts.
Other all band / mode WD receive sites use other CPUs and/or banks of CPUs.
I run and extensively test installation on the Pi (don't use the new bullseye OS which includes a version of bash with a memory leak) and Ubuntu 20.04 LTS.
WD will use a RaspSDR and was running at one time with a Flydog. But the Flydog has a miserable RF front end (and other problems as well) and results with it were mediocre.
There is code in WD which can be configured to use the SoapyAPI which on my TODO list just below debugging FST4W support. The functionality will depend upon what Soapy offers.
73,
Rob