I would if I could. However, the dropdown list for payload formats in the gateway web GUI does not mention ChirpStack v4 at all, only up to ChirpStack v3. Additionally, I checked the release notes for the newest available firmware at the RAK firmware download URL and they fail to mention any new support for ChirpStack v4 as JSON.
Nowhere in the release notes does it mention any JSON payload change or the possibility that gateways using JSON payloads need to be reconfigured as protobuf or a different JSON version to use v4. It would be nice to add a warning about this.