Oracle NVARCHAR2(4000) with AL32UTF8 possible?
If your NLS_CHARACTERSET is AL32UTF8, any VARCHAR2 column will support Unicode (via the UTF-8 encoding).  In 99.9% of cases, you'd never want to create an NVARCHAR2 column.

The NVARCHAR2 column, which uses the NLS_NCHAR_CHARACTERSET which will almost always be AL16UTF16 is really only useful in a couple of cases.  The first, and by far most common, is when you have a legacy application whose NLS_CHARACTERSET doesn't support Unicode and cannot be migrated to Unicode but where you want to support Unicode data in a handful of columns.  That's not the case here.  The second would be when the database does support Unicode but you want to use the UTF-16 encoding for some reason such as the ability to store primarily Japanese and Chinese data using mostly 2 bytes per character in UTF-16 rather than 3 bytes per character in UTF-8.  But if you do that, you're still (barring 12.1 and the ability to have 32k VARCHAR2 columns) limited to 4000 bytes (2000 characters in a UTF-16 column).  Hence the error when you try to define NVARCHAR2(4000) which would need at least 8000 bytes to store 4000 characters.