As you only have 4 layouts the formula should be "round(random(3)+1)".
I've tested the setup as you have, it works.
I made 4 blank layouts, just putting a text object in them, and setting its text to the layout's name.
When actualising/reloading the preview, I obtained a different layout on start of the app.
The code we are talking about should be sitting in the first layout that's executed.
It should work.