He doesnt count as a pure 400m runner, but Donavan Brazier has the range to run multiple 45.xx FAT splits for A&M and while he has not done that in an open (only has 2 400m races listed on world athletics, both in the low 47 range from 2016/2017) I think it is pretty clear he had the capability to get into the low 46s if not a hair faster. He has also run 3:35 for 1500.