If you remember from this post, I posited that the missing T4 scenarios were not actually missing but rather locked out due to the nature of the T4 zone control system. Since our guild's move to the more populous Badlands, I've had the chance to see these normally locked zones become contested and note which scenarios are associated with them. So here is the list I have compiled so far:
Dwarf vs. Greenskins
Kadrin Valley - Gromril Crossing
Thunder Mountain - Thunder Valley & Logrin's Forge
Black Crag - Howling Gorge
Empire vs. Chaos
Chaos Wastes - Maw of Madness
Praag - Battle of Praag & Grovod's Cavern
Reikland - Hills of Reikland
Hish Elves vs. Dark Elves
Caledor - Caledor Woods
Dragonwake - Serpent's Passage & Dragon's Bane
Eataine - Blood of Blackcairne
[EDIT: 1/7/09] Since this page gets a lot of hits, I thought I'd update it with confirmation that the Caledor and Eataine scenarios are, in fact, correct. [/EDIT]
After looking at this list, there does seem to be some missing T4 scenarios, after all. Fragendorf Castle and Skaven Caverns don't seem to be associated with any zone. So I have no idea if they are actually playable or not yet their associated quests still linger on in the game.
A mystery for another day, it seems.