I use a Mac and when SQL*Developer v4.0 EA1 is not running, then the icon in my dock is the standard one I would expect for SQL*Developer... but once I fire it up, then it changes to a square white icon with a Java logo. Needless to say this makes it hard to find, and it has never done this under v3, v2, etc.
MacOS X 10.8.4
Message was edited by: gclough Updated Title to reflect problem still exists in EA2
and your good to go