I thought I had this set fine, however now occasionally emacs will go and insert an 8 character tab, or a 6 or 5 character tab....
Maybe this is caused by the document (this has occurred in a large text file which contains many different tab spacings, though mostly 4 character space increments or regular tab characters).
(setq-default indent-tabs-mode 0)
(setq-default tab-width 4)
(setq indent-line-function 'insert-tab)
The above is the code from my init.el, which leads me to wonder why it is occasionally failing and inserting 6 character tabs.