Two things:
1. Yes, the Newbury Big 4 ARE that talented.
2. No, thousands of American HSers today CANNOT or WILL NOT ever run sub-4:00 miles, unless widespread genetic modification occurs.
1. You need to think of the Sahlmans and Youngs more as pre-eminent 2-milers and 5K runners first, milers second. That's where their collective and individual talent really shines historically. Yes, Colin raced the 4th fastest prep mile ever this year, as well as low 1:48 in the 800, but if he had given the same attention to the longer distances this Spring, he could have likely challenged Verzbicas' two-mile record while breaking Rupp's 5K mark. Meanwhile, Aaron ran 8:01.7 indoors (right about an 8:40 two mile)- one of the fastest indoor marks ever and arguably superior to his 4:01 mile outdoors. Meanwhile, Lex ran the 2nd fastest prep 5K ever this Spring in 13:43, and Leo would have arguably finished a second or few ahead had he been healthy and entered in the same race. And Aaron, Lex, and Leo still have a year of HS left.
All that on the reported 60-65 mile regimen Brosnan has these kids on. Yes, I'd say their core four (plus Nico) are that talented individually.
2. Peach Pit, we've covered the mile question already in my previous post on the topic. No, there isn't a single '4-minute mile' gene on the human DNA strand just waiting to be activated in thousands of American schoolboys if only they were just steered into and properly trained as distance runners for a few years. Multiple nature and nuture factors are necessary for 4-minute mile achievement, including under-valued talents like mental toughness, bio-mechanical efficiency, ability to recover from sustained pounding for months and years on end, ability to focus on a single goal to the exclusion of practically everything else for years, a financially and emotionally supportive household, superior coaching, safe and conducive training places, etc. etc. etc. The odds of all of this happening in conjunction with a remarkable innate talent for stride length + turnover rate are very low...so low that still just a handful have ever accomplished sub-4:00 in the U.S.
Even if you got geometrically more kids in the U.S. to take miling seriously, I doubt if you'd more than double the number of four-minute milers each year. Look at East Africa. Combine Ethiopia, Kenya, and Tanzania and you have about 200 million people. War-torn and economically disadvantaged, no doubt. But talk about a talent base and motivation! Where a single Diamond League 1500 or international marathon win could to a year's salary or much, much more! And how many combined legitimate 18-years and under sub-4:00 (using the 1500 time equivalent) milers do they have? Maybe 30-40? Or how about Japan and its 125 million relatively rabid marathon fans? Maybe 10?
Using your logic, there could be thousands of 26 ft and 52 ft HS long jumpers every year, or 7'3'' high jumpers, if only enough basketballers and footballers were enticed into track and field. Thousands of 20.2 boys 200 guys, or 52.0 400 girls. Thousands of 18' and 14' pole vaulters. But guess what? Having attended an inner city school myself back in the day, just about every baller also gave sprinting and jumping a whirl too, for bragging rights if nothing else. But after a few semesters of P.E. or a few track seasons, those who could pretty much did instead of the reverse.
Perhaps the only factor that will increase the number of HS milers from say 0-2 each year to 5+ is advanced shoe technology. On another recent post, I compared the progression in high school performance (both boys and girls) across every widely contested HS event across the last three decades, using Athletic.net statistics. What became immediately evident was that- while marginal yet uneven progress could be seen in such events as the sprints, little was clearly evident in other events save for the girls pole vault (as it is still a relatively new event). The outliers? The 800s, mile/1600s, and 3200s. The longer event, the more remarkable the progress (I wasn't able to access comprehensive national data prior to 2000, save for California State, but there similar trends also emerge).