It was never said that only the fourth could use the technique only that he invented it and mastered it meaning he was the only one who could do it alone. Minato passed the technique on to his special guards, Genma Shiranui, Raidō Namiashi and another shinobi, however all three's powers combined are needed to perform the technique. To do so, they form a triad with one another touching each others' index, middle and thumb fingers. This not only completes the technique but also, allows them to seemingly transport anything within the confines of the ring, without the need for direct contact. As they are now the guards of the Fifth Hokage, Tsunade has been marked with the technique formula.