Let $A$ and $B$ be any two arbitrary events, then, which one of the following is TRUE?
1. $P (A \cap B) = P(A)P(B)$
2. $P (A \cup B) = P(A)+P(B)$
3. $P (A \mid B) = P(A \cap B)P(B)$
4. $P (A \cup B) \leq P(A) + P(B)$