Because #CH_4# is the formula of the methane molecule.......i.e. a carbon nucleus can form up to #4xxC-H# bonds......
Explanation:
#"Methane"# is the first of a series of the homologous alkanes, general formula, #C_nH_(2n+2)#. The next member is #"ethane"#, #C_2H_6#, then #"propane"#, #C_3H_8#........