The answer lies in this picture. How many bonds are created between the carbon and the H? Well, since we have one carbon and four HS, there are four bonds. So that means we need four hybrid orbitals. So that means we have to use the two x and all the three P, x's, y's and Z's to form our four hybrid orbitals. In other words, for hybridization to take place, the two S must combine the two PX that must combine the two PY and the two PZ.