Once again, we've been seeing the rate of sub 4's go up for a couple decades. I'd say right around 2006 it started to explode, and continued picking up every few years. I added up the number of new sub 4 runners each year in 5 year intervals. Here they are, starting with 1950-1959: 1 -> 13 -> 17 -> 42 -> 26 -> 38 -> 31 -> 35 -> 28 -> 33 -> 65 -> 96 -> 119 -> (probably 200 from 2020-2024, don't have the last year yet)
So for we saw a crazy number of 42 between 1970-1974 but then regressed back to the 20's and 30's from 1975-2004. Then US distance running started to explode with 65 in 2005-2009, a new 5 year record. Then beat it again from 2010-2014 with 96. Then broke that new record from 2015-2019 with 119, and now we're beating it yet again with what will probably be around 200 from 2020-2024, we're already over the 119 mark with a year to go. It's not all because of shoes. Maybe a small percentage of the increase is due to shoes, but we've been trending like this for about 20 years.