Apple - Forcing/Getting app to run on lower Mac OSX version

NSBundle and CFBundle aggressively cache the contents of application bundles to improve performance. Although you edited the Info.plist, the Finder and Launch Services don't yet know that the application is any different.

Running the following command in Terminal to touch the application bundle will force the Finder (and Launch Services) to re-register the application, thereby noticing the changes you made to the Info.plist:

touch "/Applications/My Cool"

After running that, deselect the application, then reselect it in the Finder, and then try launching it.

Note that this won't necessarily guarantee that you'll be able to launch the app. The developer would need to make sure they set a deployment target of 10.4 and/or use weak framework linking to assure that the app will be fully backward-compatible.