If you want to fly your Phantom4 RTK over unpopulated areas, it will fall under the open subcategory A3. https://mobilit.belgium.be/nl/luchtvaart/drones/nieuwe_regelgeving/categorie_open
If you also want to operate in residential areas, it will fall under open subcategory A2.
For Belgium:
For A3 you’ll need to follow a free online course and take an exam online. For A2 you’ll need to declare to have followed a practical training. Additionally, you’ll have to take an extra written exam at the CAA in Brussels.
More information can be found on the site of the CAA: https://mobilit.belgium.be/nl/luchtvaart/drones/nieuwe_regelgeving/categorie_open/piloot)
For the UK:
You’ll need a flyer ID to operate the Phantom 4 RTK. You can obtain this by taking an online exam at the CAA:
https://register-drones.caa.co.uk/drone-code/getting-flyer-id