The development of the motherboard was started before the release of the Raspberry Pi 4 CM.
Since the format factor and connection of the Compute Module 4 have been changed, a simple exchange is not possible.
With the CM3 we have long term experience and some of our early prototypes have been working for years without failures.
The additional CPU power and large RAM memory to the detriment of power consumption are, in our experience, not necessary for use as a pure smart home system. (For other tasks like network storage, Nextcloud, virtualization, however, they are…). Another important aspect is the fact that the software for the CM4 is not yet as mature as the software for the CM3.
In the long term, a version with CM4 support is planned.
The module interface will definitely remain compatible to allow an easy upgrade.