Hi @Kacper
Supporting GSM is not planned for RPi4. The issue is manifold, but the dominant one is that it's not possible to support multiple GSM modules at the same time. Android has to be adjusted to support one specific module. We do offer custom images and it's something we could realize, but the effort and thus the cost could only be justified when used on some hundred devices.
For a personal project I would recommend to try to get internet access through Ethernet; that's plug and play. I never tried, but there must be GSM modules that have an Ethernet connector, or maybe use a GSM USB dongle with some usb to ethernet adapter. But that's really just brainstorming and something you need to look into, if and what is compatible.