In the case of Thorium (A = 232 and z = 90), we obtain an isotope of Lead (A = 208 and z = 82) after some radio active disintegrations. The number of β particles emitted are respectively

Decrease of mass number = 232 - 208 = 24
Number of α-particles emitted$=\frac{24}{4}=6$
Due to emission of 6 particles, decrease in charge number is 12
But actual decrease in charge number is 8.
Clearly 12 - 8 = 4 β -particles are emitted