How can I disable the notifications shown at bottom right corner in pgAdmin 4?
Changing the value in Query info notifier timeout to -1 it will disable it. This is documented in the official pgAdmin documentation in the Preferences Dialog help as:


  Use the Query info notifier timeout field to control the behaviour of the notifier that is displayed when query execution completes. A value of -1 will disable the notifier, and a value of 0 will display it until clicked. If a positive value above zero is specified, the notifier will be displayed for the specified number of seconds. The default is 5.