Apple obviously believes the Intel 4000 can support the retina display just fine, the 15 inch retina pro is running on only the Intel 4000 most of the time.
It's never going to be a gaming powerhouse, but that's always been the case for the 13".
For web, spreadsheets, word proc and HD video playback, the Intel 4000 is just fine on the 15 inch and will be equally fine on the 13 inch.
Of course anyone wanting to buy one for the start of school may be out of luck, since this should outsell the 15-inch rMBP buy a wide margin and there's still a 3-4 week wait on those.