You should search that ocijdbc11 is installed in a path similar to C:\Oracle32bit\product\11.2.0\client32bit\bin and that this path (or C:\Oracle32bit\product\11.2.0\client32bit) is in your java.library.path.
If you click on Help|About and the Extensions tab you will see a list of some of these.
Click on the Export button and copy the contents to a file or clipboard. Open the file (or Ctrl/V) in an editor and search for java.library.path. This is the list of ;-separated folders that SQL looks in. Ensure that this list includes the one where your ocijdbc11 was found.
But I am trying to use oracle 12 (64bit), not oracle 11. Is SQL Developer 4 not possible to configure for oracle 12 at the moment?
There seems to be no file called ocijdbc11 anywhere in the oracle 12 installation folder (including SQL Developer 3.2.2 and 4). It feels strange to need to install oracle 11 in order to get SQL Developer 4 to work with Oracle 12. And as I mentioned, SQL Developer 3.2.2 works fine with Oracle 12 and jdk 7u25. So if SQL Developer 3.2.2 works, how can something be missing for SQL Developer 4?
I am on win 7 64bit and just switched from SqlDeveloper 3.2.20.09 to 4.0. I was having the same problem as in the original post and while researching I found these posts and although they are old I thought I'd post the solution that works for me.
In Sql Developer 4.0 under tools/preference/database/advance add the path for tnsnames directory