People get way too caught up in terminology.
A quick looks at a dictionary reveals the following defintions of master:
* To conquer or overcome: (to master one's pride.)
* Become adept in: (to master a language.)
* A worker qualified to teach apprentices and to carry on a trade independently.
* A person eminently skilled in something, as an occupation, art, or science.
Applied to TKD, Master means any and all of these. Master doesn't mean you know it all ( I believe that term is "Omniscient" ... basically God). Don't go elevating anyone to that stuatus quite yet.
I have my masters' degree, but definitely don't know EVRYTHING there is to know im my field.
And knowing a syllabus... such as the ITF's, still doesn't mena you have learned "all" there is to learn anyway.
So, master (to me) means someone who has achieved a highly expert level of proficiency. But it can't be measured in terms of something as concrete as years of service.
If the ITF considers dans 4-6 "expert", then to me that should translate to master. Doesn't mean you know it all, but as the definition of the word suggests, you have achieved at a very high level.
I also agree with the thought that the term Grandmaster doesn't really need even exist. Is not simply saying somebody is an 8th or 9th dan enough? It's just more uesless terminology to me. Suddenly it makes somebody who is a "master" "less" of an expert. Splitting hairs IMO.