The moment I saw this thread, I hoped to god it wasn't another "can mary cain run a sub 4 mile" thread. In case you haven't noticed, the world record (although admittedly doped to the gills) was set when that girl was like 19 or 20. Something like that. Mary Slaney was just as good, if not better than, Mary Cain at this age. And if you look in the record books, mary Slaney doesn't have the world record.
So honestly, no, Mary Cain will not break the sub 4 barrier. She may threaten the world record at some point, maybe even break it, but not by 10 seconds.
And look at progressions from other of Salazar's athletes. Rupp's best mile in high school was a 4:01. He later dropped to a 3:51. That's 10 seconds, folks. And he might not be a speed athlete, you could argue, but if he ran a 4:08 in high school and dropped 25 seconds, he would have the world record. Plus Mary Cain is obviously an early maturer. You don't look at those 10 year olds running sub 20 5ks and say that them dropping 7 minutes could happen in another 15 years.