Math.random() for certain numbers

I have a function that creates a Movieclip every two seconds. I want each Movieclip to go to a random frame. But if there is already a Movieclip on the stage that is currently on frame 4 (for example), then no other Movieclips can be on frame 4 until the other Movieclip on frame 4 is removed. How would I do this? Thank you in advance.