I have been using pgAdmin4 9.0 for a few days now, and wanted to provide some feedback.
Overall, I like the changes. It took me a couple of hours to get used to the changes, and at first I thought there might be use cases where one might need a side-by-side interface, but after using it... I can't think of any.
I find that "Let's connect to the server" screen an annoying distraction. It can just be ignored and it goes away if you click on a different tab, but from a User Interface point of view, I think you should only get that screen if you click on the 'Query Tool Workspace' and there are no windows open.. Otherwise, it should probably just go to the adjacent tab as it did before the change. If I have windows open, I am already logged into a server, and I am not going to go through another login... Especially since I probably have my credentials saved in the 'Default Workspace' tab, so it's much easier to just expand a server and open a new query window than it is to fumble around typing server names and passwords.
As always.. Thanque for making pgAdmin just an excellent tool!