The 25-year-old, who this week formally qualified as a veterinarian at Glasgow University, produced one of her trademark late surges on the closing straight for a time of 3:59.30 as the USA’s Shelby Houlihan grabbed a surprise victory in a world lead of 3:59.06.
Elsewhere, in-form Reece Prescod, who finished third, ran the second-quickest 100 metres ever by a British male in a wind-assisted time of 9.88 seconds.
Meanwhile Mark Dry could sit out the rest of the year after coming 11th in the hammer at the Halle Throws International in Germany.
With Scottish rival Chris Bennett pulling out injured, the Commonwealth Games bronze medallist threw 70.04 metres, Poland’s Pawel Fajdek taking victory in a world-lead of 80.70m with Nick Miller third.
The prestigious event might be the last of Dry’s season – or indeed his career – as he looks at a potential hip reconstruction surgery to repair the damage which has left him struggling to walk.
Scots rivals Jake Wightman and Chris O’Hare will go head to head in today’s Westminster Mile in London.
“After a month at altitude I hope to be in the sort of shape to run under four minutes,” said Wightman, who landed 1500m bronze at the Commonwealths.
“With the passing of Sir Roger Bannister, this year it would be nice for that to happen. It will be added motivation.”