The preferred way of installing Ubuntu 22.04 on a Raspberry Pi 4 is via the rpi-imager
tool.
Please either install "Ubuntu Desktop 22.04 LTS (RPi 4/400)" or "Ubuntu Server 22.04 LTS (RPi Zero 2/3/4/400)"
as these are the 64-bit versions (marked as "64-bit" in the description). You can find Ubuntu 22.04 under
"Choose OS" -> "Other general-purpose OS" -> "Ubuntu".
After logging into your Raspberry Pi 4 you can follow the normal Ubuntu 22.04 instructions to install gr-iridium on Ubuntu.
The Ethernet interface of the Raspberry Pi 4 produces interference inside the Iridium frequency band. The device in general also emits noise which an active antenna easily picks up. Try to place your active antenna away from your Raspberry PI 4 to avoid interference.