HD Graphics 4400 is 20-30 %% better than HD 4000
HD Graphics 5000 is 50% better
Iris 5100 is 125% better and
Iris Pro 5200 is 150% better and it has addition ram cache of 128MB
Where do those figures come from? If they're from Intel's charts, they're most likely exaggerated. The Macbook Air reviews showed graphics performance about 20% better, and that had the HD 5000. I'd love to be corrected here though.