Assume we have $4$ different components as : $\{P_1,P_2,P_3,P_4\}$
These components are sequentially assembled in two different ways implies that all permutation of these components would not give the desire result.
So bind two components : $\{(P_1,P_2),(P_3,P_4)\}$
Working sequences can be find as : $\large\frac{4!}{2!\times2!}=6$
which are as follows :
$\{P_1,P_2,P_3,P_4\}$
$\{P_1,P_3,P_2,P_4\}$
$\{P_1,P_3,P_4,P_2\}$
$\{P_3,P_1,P_2,P_4\}$
$\{P_3,P_1,P_4,P_2\}$
$\{P_3,P_4,P_1,P_2\}$
Idea is much more similiar to serializability concept , here I assumed that $P_2$ cannot be executed before $P_1$ and $P_4$ before $P_3.$
Now,
Probability of getting working sequence : $\Large{\frac{6}{4!}} \ =\frac{6}{24}=\frac{1}{4}$
