It is much easier to run a 4 minute mile on the treadmill.
End of story.
The treadmill is doing mechanical work for you (paw-back) not to mention the fact that there is no air resistance. Think about it, every time you leap (flight phase of running stride)the belt is spinning beneath you covering more distance than you would outdoors with each stride. No air resistance is one part of improved distance with each stride, but as one poster joked, you can also send yourself airborne (vertical motion) and the displacement of the belt horizontally will still be greater.
I would imagine any runner who can break 4:20 outdoors can run a 4:00 on the treadmill. Probably even runners under 4:30.
2 examples: I have a teammate that ran an hour hard on the treadmill, he ended up running 12.1 miles in an hour. His last 10k was something like 30:15 because he started out slower (around 11 to 11.5 miles an hour. His last 0.5 miles was at 15 mph. He cannot break 2:00 in the 800, his best is about 2:03, and his 10k PR that spring was 32:25.
I once ran a 5k after running 5 miles in 30 minutes (to make my legs tired) in 14:31 on the treadmill. My outdoor PR for a race is 15:28.
The difference is possibly ~10%. I will conclude that a 4:24 miler could run 4:00 on the treadmill.