Hello RB211,
You haven't really given me enough information to try and help you. Do you know which version that you are trying to compile. And which operating system are you running on the Raspberry pi 4 is it Linux ( Raspbian ) or windows 10 It is rather strange because that error should be a windows only error but I suspect you are running Linux.
Please can you try and compile from the develop branch of this repo
https://github.com/captnfab/PianoBooster/The latest code for now can be downloaded using this link
https://github.com/captnfab/PianoBooster/archive/develop.zipSo please try again an please let me know if using the latest source code makes any difference.
Also I am very curious as to why you have chosen the Raspberry pi 4 is it anything to do with education? Unfortunately the Raspberry pi is rather under powered especially when also trying to run a sound generator as well (fluidsynth). But we will see how you get on. Do you have access to a PC and does it work for you on that?