Hi and welcome to Arturia forums.
This FAQ is about uninstalling Arturia products:
https://www.arturia.com/faq/installation/uninstall-remove-a-softwareTo keep a install simple you can just keep the default install paths in the installers. Some things you can change if you wish.
Which plug-in you use may not be so important. If you can do what you wan't with VST2 or with VST3 then it does'nt matter if you use one or the other. Some applications out there does'nt support VST3.
There are many informations about all this on the web. And the answers can depend on your equiptment and usage. That include how to use the differnt plugins in a certain DAW.
No matter what, then the only way you can be as sure as possible to keep a certain sound is to create a audio track. Plug-ins can suddenly change in many ways.
But a new AL should replace an old AL. Are you sure this is'nt the case? If not, then you can check the following questions. Are your install path the same for you AL4 as it was for AL3 and does your DAW read the path?
I would not recommend you install your Arturia Applications on a external drive. You can install some things on external drives, but you will not save that much space on your drive, and it may be more difficult for you to administrate. And depending on your equiptment and setup it might have a negative impact on performance. It's a matter of choice, equiptment and knowing what you are doing. It's a long topic, and it also depend on your usage.