tl;dr: After the fall 2019 firmware/bootloader update, the Raspberry Pi 4 can run without throttling inside a case—but only just barely. On the other extreme, the ICE Tower by S2Pi lives up to its name.
Three options for keeping the Pi 4 cozy: unmodified Pi 4 case, modded case with fan, and the ICE Tower.
A few months ago, I was excited to work on upgrading some of my Raspberry Pi projects to the Raspberry Pi 4; but I found that for the first time, it was necessary to use a fan to actively cool the Pi if used in a case.
Two recent developments prompted me to re-test the Raspberry Pi 4's thermal properties: