The main issue is with Poser finding the !DAZ directory. If you install 4.1/4.2 outside of the Poser install directory.
 
You can install to wherever you like without issue by pointing the installer to a directory you choose and you can select "Poser files" (doesn't selecting DAZ Studio as the application mean that you could end up with DS formatted files and directory structures instead?). The issue with the installer refusing to install the files anywhere other than in the main Poser directory is that it looks for a poser.exe file.

As said, if you create a new directory where you want to install 4.1/4.2 and in it create a file called poser.exe (as said, it can be an empty text file for all anyone cares) then the 4.1/4.2 installer will then happily install the files in that directory.

Poser will look for the !DAZ directory firstly at Poser root level and then, by deep searching, go through all you runtimes. You can leave the !DAZ directory where the installer puts it in your non-Poser directory but loading a figure will take more time because of the deep search. If you move (or copy and rename the original) from where the installer puts it to the corresponding directory location within the main Poser directory then the search will find it a lot quicker resulting in the figures loading a lot sooner.

Hope this helps...