wizzard control: run javascript on page load for only particular activestepindex

I have a wizzard control.
I am using onload in the body tag of the markup to make some javascript run when the page loads. But I only want it to run if the fourth wizzard step index is active as it is pertinent only to an object that is loaded for the 4th step.
I tried this in the javascript but no go:
if (form1.wizard1.activestepindex = 3) {bla bla}
What is the best way to do this?
 
Thanks,
Sam
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