After 3d-sub level is completely filled the differentiating electron enters into.... sub level.

Follow (n + l) rule (or) moeller's diagram.for 3d- subshell (n + l) = 3 + 2 = 5for 4s subshell = 4 + 0 = 4for 4p subshell = 4 + 1 = 5for 4f subshell = 4 + 3 = 73d & 4p subshells have same (n + l) value but 'n' value in 3d is lesser hence the energy 3d < 4p.