Here is the statement from the Windows 10 IoT Core development team: Microsoft is excited by the new RPi4 platform and is evaluating the numerous changes. Given the need for a new Raspbian OS, we do not expect the existing FFUs for Windows 10 IoT Core to be compatible with this platform.
if we want to use the raspberry pi 4 for IoT and physical computing, we are going to have to use Rasbian Buster, code in Python, and use a web based UI (even if we intend to install a local display on our device.) that will send you down a path that will at least have long term support.
.net core / xamarain for linux to run apps on Rasbian seems like something that might have dubious support / path forward in just a few short years.
Microsoft is a huge self promoter . if they were planning anything with raspberry Pi 4 they would have been telling us so for the last 6 months.
not trying to be disrespectful in this posts, just realistic