Yes, Scott is right.
Here is the way I solved it:
I installed the tab plugin with the CKEditor4 plugin (that allows you to visually modify the CKEditor settings - in the admin panel).
Since the CKEditor4 plugin uses the CKEditor v. 4.2, I had downloaded and installed the 4.2.3 version of the tab plugin.
Just unzip it under the ....\qa-plugin\ckeditor4\plugins\
(where ckeditor4 is the folder where you've installed the CKEditor4 plugin).
Under the admin panel --> CKEditor4 --> "Other Configurations" put these 2 lines at the top :
When you'll hit the tab key inside the editor 4 spaces will be inserted to "simulate" a tab.
This is the way I solved it, even if I think it's not very "clean" (I do not like the spaces instead of the tab)