I've been trying to do this since using CTA4 and to date no tutorial has helped me and I've tried again and again to figure this out. Please help me out. What am I missing!? 
I am simply trying to create a HEAD and separate BODY and save them in the CUSTOM folders, "head" and "body". 
How do I import just a head and save it in CTA4? 
How do I import just the body and save it CTA4? 
I can save the custom character just fine, but a separate HEAD and separate BODY custom will not save no matter what I do. 

Very weird coincidence as I was just figuring this out this morning! Not sure if this is exactly your situation - you have a custom character with a 360 head and you want to save just the head to use on other characters? Have your character on the stage and click on it so it's got that box around it. Now go to Content Manager> Actor> Custom and open the "head" folder. At the bottom of the column you'll see a down arrow and a "plus" sign. Click the plus sign and it will save your head - you'll have to rename it. Now if you want to apply that head to another character, all you have to do is put the new character on the stage, highlight the character and click on the saved head in Content Manager; it should apply it to the character. Hope this was helpful and actually addressed the problem.

Bless your beautiful soul!!!!! You are a genius! 

It's so funny, because that process is exactly how you do it with custom MOTIONS... 

Thank you! You are no longer a padawan but a master JEDI!!! 

You unlocked the mystery I could not figure out! 

NOW. A follow up question. This same process DOES NOT WORK for the CUSTOM BODY folder? Why??? How to get custom body into the custom body folder. 

Trying to make switching outfits and hair styles as easy as possible for one custom character. 

Able to help you, glad I was, yes.....lol.
76% of original size (was 665x19) - Click to enlarge
That is quite curious about the body folder, but I would just save it as a custom character; you can set up separate folders in there to keep all of your characters organised. The heads are easily interchangeable when needed.