First, there are no licensing issues whatsoever. We always offered a single license for all the supported platforms so, your existing Serial Number will still work in P3D V4, regardless if you want to install it together with other sim, or in an entirely different PC.
Yes, it's possible to install into P3D 3.2 and 4.0 at the same time. The installation will migrate to a single shared folder which you CAN choose (by default C:\Program Files\Addon Manager will contain everything FSDT), so you'll have a single installation of all our products in a single folder, shared by all the different sim, which result in significant space saving compared to the previous installation method.
However, P3D 3.2 has several bugs in the handling of the new add-on.xml package method, so we use it only in P3D 3.4 and 4.0. It's not really an issue, just that sceneries will still use the scenery.cfg file, and modules like GSX will still use the dll/exe.xml files, so you'll lose the ability to reinstall the sim without reinstalling the addons, unless you first upgrade to P3D 3.4.
In P3D V4, we'll use the new add-on.xml method in any case.
