I've gotten lucky with shards and am looking at the t4 castle on a shard pretty early - however, I don't know if by taking this, will it cause all the AIs to suddenly have t4 units as well? Does the AI unlock stuff at the same time as me, or are they gathering random shards off-screen?

I want to know that too because it SEEMS like the moment I unlock a higher tier unit suddenly all enemy masters have them too.
Not only that, suddenly rebels, attacking inquisitors have them. And neutral guards and dungeon creeps get a significant power boost (those always had access to higher tier units but now they have more of them)

I've seen it many times that the AI players have higher tech than me (e.g. they have Catapults in their armies when I don't have access to them yet). So the tech progress is not the same for all players.

I think I've encountered masters who have more tech than me, so that would seem to imply that their progression is independent. Now I'm not 100% sure of this because I've been playing a lot of skirmish as well as the campaign so while I'm pretty sure it was in the campaign it might have been skirmish.

EDIT: Ah, pawels-aow beat me too it ;)