Mass Spectrum and Molecular Ion peaks
The major isotopes of chlorine are 35 and 37.  If 4 chlorines are present in a molecule, then the following isotopic combinations of chlorine are possible


35, 35, 35, 35
35, 35, 35, 37
35, 35, 37, 37
35, 37, 37, 37
37, 37, 37, 37


So there will be 5 molecular ion peaks.  You could calculate their relative heights by factoring in the relative isotopic proportions, along with the number of permutations of each of the above combinations.