The first model of the RPi4 seems to have an issue with USB-C, as reported by ArsTechnica in July 2019. The effect is that E-marked cables do not work well.
The Raspberry Foundation has confirmed this issue and wanted to provide a fixed variant in "next few months". That was in July 2019 and a few months are over, so I wonder whether I could distinguish "old" RPi 4 models from "new" RPi4 models.
How do I distinguish RPi4 boards with USB-C issue from the same model with USB-C issue fixed?
Revision 1.2 seems to contain the USB-C fix. Raspberry Pi 4 models with the fix have reached the market. The difference can be found in hardware and software.
Regarding the hardware identification, it's easiest to see near the SD card:
For software, the Raspberry Foundation lists Revisions for the RPi 4 rev 1.2 like this:
b03112 4B 1.2 2GB Sony UK c03112 4B 1.2 4GB Sony UK
Raspbian kernels seem to be fixed since end of November 2019.
Thus the output of
dmesg | grep "Pi 4 Model B Rev"
should result in
[0.000000] OF: fdt: Machine model: Raspberry Pi 4 Model B Rev 1.1
or
[0.000000] OF: fdt: Machine model: Raspberry Pi 4 Model B Rev 1.2