How to watch Decker season 4 and onward in Europe?
VPN of your choice (either on your web browser get yourself an extension like Urban VPN or, preferrably, it is worth it subscribing to a VPN service like Mullvad VPN or ProtonVPN) → set it to the US → Decker on Adult Swim for free!
If all else fails, go to ext.to and search for Decker.