AI changes I have seen in 1.4:
Better planet management. This is possibly the biggest change. They still need to figure out about farms, though, as well as slightly less extreme specialization. Still, when I capture developed planets from the AI I find a lot less I feel I have to destroy and rebuild.
Better fleet management. The comments above about the difference between offensive and defensive fleeting are accurate. The improvement is in attacking fleets, so far.
More commitment to war efforts once war is declared. No declare war and give up immediately. They can still be slow to prosecute the war, but then again, I got seriously jumped in one declaration and they sniped out a lot of small ships, civilian traffic, and starbases in the very first turn of the war. This seriously affected my range and reach, requiring sending constructors to catch up with my avenging fleets so I get somewhere and start avenging.
Better patrolling of borders for when I was sending constructors to set up frontline starbases stealthily. I am sure it was mostly wandering and not something totally deliberate, but it was certainly more effective.
Better sniping of vulnerable ships. Warships with low HP and unprotected transports/constructors are jumped on with enthusiasm. I thought I had one fleet of unprotected transports far enough out of action, but they got sniped. I am convinced the enemy ships jumped out of where they were hiding in a nearby shipyard, came out of the shipyard, and struck the vulnerable fleet in one turn. Ouch.
Hiding tons of ships in shipyards rather than planets. This may be an old behavior, but it was newly noticed by me. And you don't know how many ships are hiding there until you actually attack the starbase. I think they should get military starbase benefits for being stationed in the starbase itself. Some of my most entertaining battles have been fights over shipyards.
Adjustments to my weapons and defense choices. Several times, they have caught me off guard with new selections of armaments. Then again, I do it to them as often as I can manage. One game actually required designing and building fleet augment support ships to gain military domination. That was very impressive.
A little more diplomatic awareness and less exploitable trading patterns. Good foundation for more changes to come.
Better display of tactical behaviors in the battle viewer. This is probably not an AI function, but it adds to the overall impression of intelligence when they aren't doing seemingly brain dead things flying about and such.
Contrary to an earlier post, I have seen AI on AI action. One game had three different two-party wars in the diplomacy screen and none of them involved me. The Bloodlust event had happened twice in a row! It was a hostile galaxy all the way around and I did get dragged into the mess later. In another game, playing as Altarians, I got a faction that suddenly surrendered to me because another faction was pounding on them mercilessly. That really turned that game around. I had been blocked in by three different factions spawning next to me. When one of them surrendered to me, it gave me access to the rest of the galaxy.