One of the dramatic improvements in the Pi 4, that I haven't seen discussed too much, is the closer-to-true gigabit ethernet capability. In my particular use case with Stellarmate, this is perhaps the biggest improvement I could have hoped for. As I understand it, the Ethernet and USB shared the same bus on the Pi 3, but the Pi 4 has dedicated busses for each for much faster throughput.

I was always less than pleased with the client-server file transfer time for focus, platesolving, etc., so I now use Powerline Ethernet Adapters over an extra power cord out to my scope. On the Pi 3, this dramatically improved the file transfer speeds to make those functions tolerable...but still not great. With the Pi 4 and it's truer gigabit speed, it should be lightning fast. I can't wait for SM OS on the Pi 4!