The men take pride of place by virtue of crossing the line in 38.70, the third-fastest junior performance in history, only bettered by their compatriots at the 2004 and 2012 championships.
Jalen Miller produced a sold first leg and handed off to 100m silver medallist Trayvon Bromell, who ran a terrific back straight to give the USA the edge over an inspired Japan.
Kendal Williams, the 100m winner, had a slightly ragged bend but still handed over in the lead to 200m gold medallist Trentavis Friday who showed he could shine on Saturday as well after his feats the preceding night with a storming last leg that took his team clear of their Japanese rivals.
Japan came home second in 39.02, just 0.01 shy of their Asian junior record that was set at the last championships two years ago in Barcelona.
Kaylin Whitney anchored the USA home in the women’s 4x100m with a world-junior-leading 43.46, the eighth fastest junior time in history and the third quickest in the history of the World Junior Championships.
Still 16, the Florida high school student completed an excellent week at Hayward Field, winning the 100m bronze on Wednesday then the 200m title on Friday before getting her second gold medal on Saturday afternoon with temperatures edging over 30 degrees Celsius..
Teahna Daniels put the US team into the lead after the first exchange, ahead of Jamaica, Germany and Trinidad and Tobago.
The hosts squad then had individual sprint finalists Ariana Washington and Jada Martin combine to give a comfortable lead to Whitney, who secured their country’s ninth world junior title in the event and the sixth consecutive one since 2004.
Jamaica’s Sasha Forbes, Kedisha Dallas, Saqukine Cameron and Natalliah Whyte were a distant second in 43.97.
No Little trips this time
Shamier Little redeemed herself after her disappointment at the last edition of the championships in Barcelona two years ago when she fell at the last barrier while in medal contention, by taking the 400m hurdles title in convincing fashion with 55.66.
The Texas A&M student, who arrived in Eugene with the fastest junior time in the world when winning 55.07 at the NCAA Championships last month, ran a controlled first half of the race and hit the second bend even with Great Britain’s Shona Richards before going through the gears.
Richards finished second in a national junior record of 56.16.
Croatian discus throwing is on a high at the moment, and Martin Markovic did what his illustrious compatriot Sandra Perkovic was unable to do six years ago when the championships were held closer to home in the Polish city of Bydgoszcz and where she finished third, and bagged a world junior title.
Markovic took the lead with his opening 64.70m and, after a foul, he unleashed the 1.75kg implement out to 66.94m, a national junior record by more than a metre as he became the third-best junior thrower ever.
Germany’s Henning Prufer followed up his world youth silver medal with another of the same colour at junior level after a personal best throw of 64.18m in the fourth round.
Rouguy Diallo took the women’s triple jump title with a massive wind-assisted 14.44m.
Diallo, who led the qualification with a personal best of 13.77m, showed her superb form from the start when landed at a wind-assisted distance of 14.24m.
After a foul in the second round, Diallo then improved to 14.44m with a 3.3m/s wind with her third effort to confirm her gold medal position. It was the second-longest leap under any conditions in the history of the World Junior Championships.
She was then rewarded with a wind-legal French junior record of 14.20m in the following round to solidify her lead.
Two-time Cuban junior champion Liadagmis Povea also had the best competition of her life, taking the silver medal with a windy 14.09m.
Bahrain finally strikes gold
The women’s 3000m steeplechase title was captured by Bahrain’s Ruth Jebet in 9:36.74.
Jebet simply ran away from the field over the last kilometre of the race to win Bahrain’s first ever IAAF World Junior Championships gold medal, and their first women’s medal of any hue.
Kenya’s world youth champion Rosefline Chepngetich began surging at the end of the third lap in a bid break the pack up but then Jebet moved up and got to the front just before the 2000m mark, which was reached in 6:26.18.
Chepgnetich and her teammate Daisy Jepkemei, the defending champion, ran away from the pack followed Jebet but had no response to her kick on the last lap and came home second and third respectively.
Earlier in the afternoon, USA’s Tre’tez Kinnard was the fastest of the three men’s 800m semi-final winners in 1:48.04, but the favourite for the final on Sunday remains Kenya’s 2013 IAAF World Youth Championships gold medallist Alfred Kipketer, who was a comfortable winner of his semi-final in 1:48.67.
At the end of the penultimate day of the championships, USA are leading the way on the medal table, the host nation having eight gold medals, four silver medals and five bronze medals. Russia and France are the next best with three gold medals apiece.
Phil Minshull for the IAAF