But that's not what Tiers mean. The Tiers mean "How many variants of your army can you take and still be competitive?"
i.e; How many units in the Codex are actually good? Must you take certain things?
As it stands, there's one (or none, depending on who you ask) way to build a competitive Tau army. And that's why its Tier 4.
No they don't. Those just happen to be things that do synergise, since the only other option is the Captain, which sucks in comparison. Its not that they need to synergise, but, it just so happens that the best things in the army, do.
You don't need Sanguinary Priests to be competitive. You only need the Librarian for the Psychic Hood, anything else you give him (by way of powers) are just gravy. And players with balls don't even take Librarians.
The most popular Librarian has buff powers. The best Librarian has debuff powers. Which Mephiston doesn't have.
Hell, take the Captain, and load up on Librarian Dreadnoughts. That's competitive too.