A planetary gear train may include first to fourth planetary gear sets having first to third, fourth to sixth, seventh to ninth, and eighth to twelfth elements, the first to third gear sets being mounted on an input shaft, the fourth gear set being mounted on an output shaft, first shaft connected to the first and fourth elements, second shaft connected to the second and ninth elements, third shaft connected to the third element and the input shaft, fourth, fifth, sixth, and seventh shaft connected to the fifth, sixth, seventh, and eighth elements respectively, eighth shaft connected to the tenth element and externally gear-meshed with the fifth shaft, ninth shaft connected to the eleventh element and the output shaft, tenth shaft connected to the twelfth element, and externally gear-meshed with the seventh shaft, and two transfer gears each forming external gear-engagement between a corresponding pair of shafts.