You could generate a random number, and use a condition that executes a given branch based on the generated number.
E.G.
Set integer [~randomNumber] value as random from 1 to 4
Begin Integer Compare : [~randomNumber] Equals 1
Run application 'C:\file1.txt'
Else If Integer Compare : [~randomNumber] Equals 2
Run application 'C:\file2.mp3'
Else If Integer Compare : [~randomNumber] Equals 3
Run application 'C:\file3.pdf'
Else
Run application 'C:\file4.jpg'
End Condition