I have used pgAdmin3 in the past and when creating an extension there is a drop down list of available extensions. In pgAdmin4 the list is empty.
I thought that maybe I had to begin typing an extension name to get a list of results, but to no avail.
TIA for help!
I did not check it myself, but the first thing that comes to my mind is - contrib
package is not installed. So the difference is not between PgAdminIII and PgAdmin4, but rather between Postgrgres installations