I've never done something like trying to combine 2 characters into the same scene until today.

If I click New or Open under File, it closes the character I had loaded into the scene. In DS4.

If I choose Merge, at least with one figure/character already in the scene, it makes her disappear and the newly opened file appear. I had 2 versions of V4 I was trying to put side by side in the same scene. Apparently however, the first version is still hiding in the scene, but invisible. I found that when I changed poses, the other suddenly appears mangled up inside old V4's body. So what's going on? I was under the understanding or expectation that the 2 characters would appear side by side in the same scene so I could transfer the hair & props from one to the other.

I'm using DS4 Advanced if that helps.

If the figure in each scene is at the (0,0,0) location, when yu merge they'll be in the same place.  You need to select one of them and move it to the side (select the whole figure and translate on the X axis).

New deletes everything in the scene to give you a 'clean canvas' to work on.

Open has a tendancy to replace one scene with another one.

I don't use those to load in a .daz scene file, if you've saved the file into a DS native directory then it will show up in your Content tab, or if your using it their not so 'Smart Content'. From there you just need to double click it to load it, or right click it for an options menu and select either load or merge, or you can 'drag & drop' it from the tab onto the viewport.

Thanks. What I've been doing is saving my DAZ Scenes in a separate folder on another drive, not only for more space, but in case my C drive ever fails, or I need to delete my DAZ3D folder in Documents or uninstall DAZ for whatever reason, all my Scene files won't be deleted with it. 

So they are on a separate drive. And also backed up for good measure on an external drive, and 2 large flash drives.  And a BD-R. 

but in case my C drive ever fails, or I need to delete my DAZ3D folder in Documents or uninstall DAZ for whatever reason, all my Scene files won't be deleted with it.

if this ever happens then your scene files wont work, until you reload all the items for each one ...........there all in your data folder in documents.

the hand book is here

http://docs.daz3d.com/doku.php/public/software/dazstudio/4/userguide/getting_started/start

Well, I know I need DAZ Studios to open those Scene files with. But not sure why all the items wouldn't already be in that saved Scene file if it was saved with all the characters, hair, poses & props and other objects. Doesn't all that make up the Scene?

At any rate, I've also got the DAZ 3D folder backed up on multiple drives separately. I'd rather keep the Scene files consolidated in their own folder outside of there that's not within another folder within another folder, within another folder, within...

The Scene file does not include the object and morph data (those are saved in the Data folder of your first DAZ Studio Format content folder) or the textures (usually in :runtime:textures).  For content provided in Poser formats, which is most content prior to Genesis, the first time you save a scene with them, the Data folder entries are created, so just reinstalling the content won't automatically recreate the Data files.

O.....K.  Well, I know this. I'm definitely keeping both the Scene files folder and DAZ 3D with all its many many many included folders like the Data & Textures. 

One way or the other, I don't want to lose any of my important content.