Making asp.net button invisible on page postback

Hi,

 I have an ASP.NET button, and it sits on a page called step3. When on step4 (obviously the next step), I want to make the button on step 3 invisible on a postback (so when pressing a button on step 4 to go back to step 3). I tried if page is postback then button is invisible (pseudo code), but that didn't work

 

Any ideas?

Thanks
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