Draw out the Lewis structure of XeF4. You'll find that it has four fluoride ligands bound to it, along with two lone pairs. That gives a total of six"things" around the Xe center, thereby requiring six hybridized orbitals. Start with s, p, and then d. So the s hybridizes, along with the 3 p orbitals, giving you four hybrid orbitals. You need two more, so you take 2 d orbitals as well to yield a total of 6 hybrid orbitals.