I've played 10 or so games of Arkham horror and have never seen the terror track go past 4. I always seem to run out of turns before it does much of anything. Especially against Hastur, when terror track matters. ( I know the King in Yellow fixes this, but that expansion is out of print right now.) His 8 clue to close a gate restriction just brings about an anemic Hastur because the terror track never did much of anything.
Perhaps I'm not raising it all the times that I should. The triggers I know of are:
Monster limit in the outskirts
Dunwich horror board monsters wandering into vortexes...
And the occasional encounter card.
So, am I missing anything? Do people feel the same way I do? Have you ever seen the terror track above 4?