By default, Flash Builder 4 will run or debug your applications in a browser.  This is just annoying.

Instead, you can make it open the apps inside the Flash Stand Alone Player. First, download the “Projector Content Debugger” (Adobe’s funny name for the Flash Player) for your OS from here: http://www.adobe.com/support/flashplayer/downloads.html. Install and run the program once to make sure your operating system knows where to find it.

Next go to the run or debug properties of your project and change the target path from /some/long/path/file.html to /some/long/path/file.swf

That’s all you need to do!