Apart from tetrahedral geometry, another possible geometry of CH4 could be square planar with the four H atoms at the corners of the square and the C atom at its centre. Why CH4 is not square planar?
The square planar geometry has a bond angle of 900. This is lesser than the tetrahedral bond angle which is 109.50. Due to this, in the square planar geometry, the repulsive forces are more and hence, square planar geometry is less stable.