The biggest is that is a really underpowered old Mac; the answer to question in title — "Slow Mini?" — is Yes. The second issue is JRiver seems to run least best on Macs, but I can only say for sure having never run it on like a new, current computer — only older computers running latest macOS, Windows, and some Ubuntu derivatives.
I am running JRiver on an old Mac. It's fine.
a 2014 higher GHz i7 or probably even i5 with 16 GB or probably even 8 GB RAM and an SSD using the current macOS would run just fine, for instance. Not that I would suggest, at this point, getting a better 2014 because the current macOS is very likely the newest and last one it will run. The HDD is probably by far the biggest culprit. macOS stopped running well on those a while ago, maybe Mavericks is last one I can think of having acceptable performance in my personal experience.
All that being said, I have the "master" license and have run JRiver on assorted older equipment and the Mac version has simply been inferior in performance to Windows or Linux version. But no matter how much I prefer to use JRiver as my music library program, I wouldn't want to ever run Windows as my main OS unless I had to, and I wouldn't ever have to because there is Linux but then I wouldn't ever switch to Linux as my main OS over Mac because I need certain commercial software.
So, in summary from my experience –
• on a Mac, at least, you need an SSD and just in general I can't see trying to use a Mac now for much of anything with just 4GB RAM. The Mac spec'd as you describe I couldn't suggest for any use nowadays.
• the program runs best on Windows, like it's still primarily written for Windows… but compared to when I first tried the Mac version and found it unusable, for quite some time now using some decently spec'd old Mac or other, JRiver performance has been totally acceptable on a Mac. Linux app also seems to run better than Mac app so you may well be better off with your Rpi4 than that Mac as currently configured.
You could also try running Linux on that Mini and seeing if you prefer the performance. The one glaring issue with Linux version was no drag-and-drop / no rearranging songs in playlists or Now Playing queue and I think I read here that has recently been resolved; not sure, haven't run Linux version in a while.