So you deleted the windows appdata for your user account, in the roaming profiles directory? There's a directory in there for sql developer, and a subdirectory for version 4 - you want to delete that and try again.
Also, try doing the startup from the bin directory copy of sqldeveloper.exe - you should get a cmd window with more output/feedback/error messages
If it's still hanging on startup, you can attach to the PID with jstack.exe (in your jdk/bin directory) and send that dump here
Roaming profiles directory. That was the culprit. Thank you, thank you, thank you!