Believe me, as a European, a sub 4 mile is well known as a high standard in athletics. Literally everyone who knows about running has heard of Bannister. It's not like the Scandinavians are unfamiliar with what a mile is.
More likely event wrote:
He's not going to find any mile races in Europe. He could maybe find one in the UK/Ireland, otherwise have to travel all the way to the US where the season for most is wrapping up. Alan Webb had to specifically ask for a mile race in Belgium.
I agree that a sub-4 mile by a 15-year-old would be a huge deal to US fans but I don't think see him going so far out of his way to do it. Most likely he will run more 1500s in North/Central Europe and then European Youth Champs.
To casual viewers, I'd say a sub 3:40 1500m is seen as the standard for sub-elite middle distance running. People know 3:3x is fast. 3:42 means nothing to most.
3:3x and 3:5x are known to be fast by casual fans (and even non-fans in the latter).
I agree he won't tackle the mile this season or probably even in the next 3/4 seasons. When he is an adult then he is more likely to run the European/American circuits and come across a mile race.