It works fine, but I had some problems getting it to boot, first I tried to use an older SD card with raspbian installed on (doesn't work), then i download noobs and flashed it to another SD-card (boots up but hangs on the colorfull boot-screen) after this I manually flashed the raspbian buster image to the SD-card (works)
The Pi 4 runs only on Raspbian dated on/after 24 June (tagged 'Buster').
I have been playing with my new Pi 4b and running it alongside a Pi 3b and Pi 3b+. So far, I love it.
The Pi 4b has a couple of quirks, which I am sure will be smoothed out in the next release or so of Raspbian. The quirks I ran into, so far, is only running it in a headless configuration.