If I have both Poser (ver 7) and DazStudio versions 3 & 4 on my computer do I select Poser or Daz Studio when installing the content? Or do I install twice once with Poser selected and then with Daz selected?

A very confused Ken.... ;-)

I install to Daz Studio and add the external runtime to the Poser 7 library. I would advise against installing multiples of the same files as it usually causes conflicts. Especially with the base Gen 4 Figures.

Yes it is possible from DAZ to import "poser type" files from the hard disks of the computer but you have to be careful because for one thing you will have a lot of duplicated items which is not a big proglem but more ennoying the process might dammage already instaled files

It doesn't really matter whether you choose DS or Poser as the target application -- you can specify the path to your shared runtimes either way.  I use DS as that way the Gen4 installers don't complain if there isn't a poser.exe in the location you're installing to.  The same files get installed whether you choose DS or Poser.