You'd have to use variables. Say you have for possible outcomes, and a twenty five percent chance of each occuring.
First, start an event and select variable operations.
Choose a new variable, call it whatever.
Now, say set to a random nuber between one and four.
Then, conditional branches.
Make one that says: If (variable) is = to 1
- whatever
ELSE
and then make a new conditional branch under the else option, like so.
If (variable) is = to 2
- whatever
ELSE
One more time.
If (variable) is = to 3
- whatever
ELSE
This time, don't make a new branch. Just put whatever you want the forth outcome to be under else.
Hope that helped you man!