I started playing the series at Civ III and it really doesn't feel like there's been much of a difference in AI capability. In the jump to 1 unit per tile in V you run into the problem with increased tactical control that while funner for the human player it becomes much easier to exploit and dominate the AI. Just compare Total War with Crusader Kings II a great example int he contrast. The other thing to keep in mind when talking about AI for a game like Civ, it's not just that it needs to be good, but it also needs to be fast. How long the player needs to wait between turns has to be taken into consideration. You could make a powerful AI that plays the game really well, but if it takes an extra minute or two between turns on an average PC then it's a failure. I'm not sure exactly to what extent this applies here, but it's gotta be a factor.