You can have DS 3 and DS 4.5 on the computer at the same time, but not 4 and 4.5, since they're the same number version if that makes sense.  There are changes in 4.5 - new file formats that makes scenes saved there incompatible with previous version.  I just changed my OS and am about to install 4.5.

As for Apollo, you do NOT need #4 to have clothing, there's lots of it in the other packs.  Anton just didn't finish some of the things he'd planned for #4.  Be sure to scroll down the download page for other items made by various people.

Installing Apollo - I don't recall doing anything special other than copying the items into the correct folders in your runtime.  You shouldn't move the Geometry or Texture files (you can, but you'd have to edit other files) but any files in the "libraries" folder can be put anywhere you want within the libraries folder.

Be sure to read the readme files that come with the packs, too.

Cheers,
Alisa

RETIRED HiveWire 3D QAV Director