I don't understand the definition of monotonic access structures
It's for $B,C \in 2^{\{P_1, P_2, ..., P_n\}}$

So, take $B = \{1,2\} \in \mathbb{A}$ as in your example. $\forall C \in 2^{\{1,2,3,4\}}$ such that $B \subset C$, $C$ must be in $\mathbb{A}$.

Possible values of $C$ are $\{1, 2\}$, $\{1, 2, 3\}$, $\{1,2,4\}$, and $\{1,2,3,4\}$. As you can see not all of them are in $\mathbb{A}$.

The idea is that if any set has access, all sets that contain it must also have access.