There is no pgadmin4 package in either the repositories or the AUR, although the AUR does have a pgadmin3 package — for postgresql administration tasks. It is however no longer maintained — the package was last updated in November 2020.
If this is the package you want, then it’ll be built from sources on your local machine, so it’ll take some time. And you must also first make sure your system is fully up to date before you do so…
If you really want the pgadmin4 version, then I’m afraid the only option right now is to either build it from sources, which you can download here, or to install the Debian/Ubuntu version by way of debtap from the AUR, but be advised that Manjaro cannot offer any support for that.
In order to install debtap, first update your system as explained above…
sudo pacman-mirrors -f && sudo pacman -Syyu
… and then install debtap…
pamac build debtap
… or…
yay -S debtap
Then, you must download the Ubuntu version of the pgpadmin4 package — this is the 7.5 version, which is the latest for the Debian/Ubuntu platform — from this location and install it using the debtap command — I presume you’ll need to do that with sudo, but I’ve never used this myself, so I don’t know.