sp3d. For each energy level (s,p,d,f) there are a certain amount of atoms in that section. S has 2. Divide it by two, you get one. P has 6. Divide it by 2, you get 3, and so on. Because selenium tetrafluoride has 5 things around it (four fluorides and a lone pair), it will go up to 5, which is sp3d.