Probability Equation for a First 4

I need some help in creating a probability equation for an exotic bet type.I have found equations for exactas and trifectas but I need an equation for a first four. An exacta is picking the first two horses past the post in order, a trifecta is the first three in order and a first four is the first four in order.

x = the percentage chance of Horse X winning

y = the percentage chance of Horse Y winning

z = the percentage chance of Horse Z winning

Here is the equation for an exacta:

$\displaystyle \frac{100}{x}*\frac{100}{100(\frac{y}{100-x})}$

Here is the equation for a trifecta:

$\displaystyle \frac{100}{x}*\frac{100}{100(\frac{y}{100-x})}*\frac{100}{100(\frac{z}{100-(x+y)})}$

Could someone please give me the equation for a first 4?