This is one of the more common error messages with Fujifilm cameras. It happens across the range, so buying an X-T4 doesn't rule out the error. In most of the cases it comes down to two issues: (a) card read/write issues - also when switching on, the camera tries to read the cards or (b) unable to detect the lens (sometimes combined with f0 message on screen).
Possible remedies: use only Fuji recommended cards. Fuji cameras are extremely susceptible to cards. I never lost images due to card issues until I started to work with Fuji. It happened a couple of times already. As for the lens, try to demount/mount the lens again.
As for the repair service: did Fuji USA repair the camera or Adorama themselves (or their partner)? I use Fujifilm Professional Service (FPS) here in Europe and have noticed that their quality differs greatly per country. I've been told that FPS USA is rather good and almost on the same level as those PS-ses from Canon, Nikon and Sony. I assume your camera is still under warranty and Adorama should solve the issue regardless of which repair service they use. Seek publicity also on Adoramas user review board.