According to the article, there is a yellow checkmark for Pi 4 for Stretch (if it is upgraded to the 2019-04-08 release).
I'm curious what limitations the Pi 4 has in this state.
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My understanding is that there is new code required to support the RPi4 and that code is only delivered in Buster. If you need Stretch, you'll need to run it on earlier Pi hardware. You may get it to work, but why bother with possible limitations and degraded operation? If you have software that doesn't run on Buster, it's time to move it forward (or ask the developer to do so).