I didnt play f4 so cant comment on that but even in spec2.0 the AI is still pretty lame I think. The AI still pretty much sticks to a single line all the time. You can see that in the video too. The cars take pretty much exactly the same line each time. Especially the GTR and F40. Seems the biggest difference with f4 is that the ''evasion script'' in GT5 is better.
You know why? Because optimal line is important in making good times, its not because AI is stupid, its because they should race that way. Watch some F1 and see their lines there