On a Saturday morning, four families arrive one after another at the entrance area of an amusement park. Each of the four families pays at one of six cashiers, whereby it is assumed that each cashier is chosen with equal probability. Describe in the context of the problem two events $A$ and $B$ whose probabilities can be calculated using the following terms: $P ( A ) = \frac { 6 \cdot 5 \cdot 4 \cdot 3 } { 6 ^ { 4 } } ; P ( B ) = \frac { 6 } { 6 ^ { 4 } }$