Actually, the first person known to have referred to time as the fourth dimension was Edgar Allan Poe, in a prose-poem essay entitled "Eureka!" first published in The Gentleman's Magazine in 1848. A later writer, H.G. Wells, spun a story of a time traveler in The Time Machine (1895) in which he announced, "There are really four dimensions, three which we call the three planes of Space, and a fourth, Time." Finally, although Albert Einstein's mathematical work provided a foundation for pronouncing Time the fourth dimension, he did not explicitly do so; that was done by Hermann Minkowski in two major papers, "Raum und Zeit" (1907-08) and "Zwei Abhand lungen über die Grundgleichungen der Elektrodynamik" (1909). The former introduced non-Euclidean spacetime, a concept that Einstein did not at first accept, though it was based on his 1905 paper. If anyone deserved the courtesy of being acknowledged as the progenitor of time as the fourth dimension in a mathematical sense, it was Minkowski, not Einstein.
Well I'll be a monkeys uncle!
