First, Beta 4 is much more like a game than any of the preceeding beta's.  It really is amazing how much can be done in a week, and that is a real testiment to the moddability of the game engine.

Now for the concerns.

The world doesn't seem "alive" yet.  It certainly is more alive than it was before, but there needs to be a bit more to do.  I think that interactive quests (ie quests that come from player or NPC created structures instead of random buildings) would help here, as well as more realistic NPC's and more events.  A lot of times I just seem to be continuously clicking end turn.

Second, it strikes me that nearly all NPC's are hired, dead, or hostile to you by 100 turns in.  There should be NPC's spawning (preferably from settlements... after all they should be coming from somewhere), and NPC's should get progressively more expensive to maintain as well as hire as they increase in level.  They should also not all be hireable.  Having a large number of NPC's going around doing their own thing and influencing events throughout the game would be nice, and add to immersion.  Really, there should always be more champions not aligned with nations than aligned.

Third, tactical battles.  Right now there is very little tactical about them.  Backing soldiers off a ways (or at least giving someone the option based on the initiative of the respective leaders) would be very helpful, as would more tactical "choices" (ie clear use of terrain, clear effects of morale, formations, etc).

Fourth, city specialization.  I like it.  A lot.  I would like to see more opportunities for a trading state though.  Caravans need to be protected, and if you focus a city on trade that should dramatically increase the contribution that city is making to your economy (I think this would go a long ways to making the one city strategy viable).