Hi @nixni
Have you checked your 4G data download when the phone downloads OK on WiFi? It might be using both. Just a thought.
Try starting the phone in [https://www.androidauthority.com/pixel-4-safe-mode-1048588/|safe mode] and check its’ WiFi performance.
If it is OK a ''downloaded'' app is the cause of the problem. The trick is to find out which one.
You will have to uninstall each ''downloaded'' app, one by one and test the phone in normal mode between each uninstall. When the phone tests OK the last ''downloaded'' app that was uninstalled was the culprit. Try starting with ''downloaded'' apps that were installed or updated about the time that the problem was first noticed.