See the official documentation (http://docs.oracle.com/cd/B19306_01/server.102/b14200/sql_elements001.htm#i54330)
Variable-length character string having maximum length size bytes or characters. Maximum size is 4000 bytes or characters, and minimum is 1 byte or 1 character. You must specify size for VARCHAR2.
BYTE indicates that the column will have byte length semantics; CHAR indicates that the column will have character semantics.
But in Oracle Databast 12c maybe 32767 (http://docs.oracle.com/database/121/SQLRF/sql_elements001.htm#SQLRF30020)
Variable-length character string having maximum length size bytes or characters. You must specify size for VARCHAR2. Minimum size is 1 byte or 1 character. Maximum size is:
32767 bytes or characters if MAX_STRING_SIZE = EXTENDED
4000 bytes or characters if MAX_STRING_SIZE = STANDARD