I think it's possible, because CEC data is merged into A/V data. RPi4B uses a different graphics driver to deal with higher resolutions and two HDMI outs. CEC signals have to be filtered out of the HDMI stream, before CEC adapter can handle it.
This is almost 100% incorrect... (sorry...)
CEC data is never part of the AV stream. In fact, CEC is a separate LOW speed interface (actually a bit like a slow I2C bus). It uses a separate line in the cable from the AV data. Most cables will include CEC support, but there are some that do not.