There are a number of world class 800/1500 runners who can split ~45.5 but <45 from blocks that's another story. Beyond 500m or so additional muscle mass is detrimental to fatigue resistance. Juantorena was certainly capable of <45 and <4 but it would have been close, especially once out of sprint spikes. (My guess is he would have preferred racing flats and an uneven pace in breaking 4.) Everett, Gray, Borz, Kipketer, Lopez and Rudisha could have been close to accomplishing both. I have no doubt that Jones had a 45 flat relay in him.