You should have a directory structure similar to this:
c:\oraclexe\apex (this is the unzipped Version 4 kit)
c:\oraclexe\app (this is created by the Oracle XE installation)
c:\oraclexe\oradata (this is created by the Oracle XE installation)

Step 1
Open DOS CMD window and change default to the directory of the unzipped Apex kit.
e.g.
CMD> cd c:\oraclexe\apex

Step 2 - Upgrade to Version 4
CMD> sqlplus system/password as sysdba
SQL> @apexins SYSAUX SYSAUX TEMP /i/

Step 3 - Set images directory
CMD> sqlplus system/password as sysdba
SQL> @apxldimg.sql c:\oraclexe

Step 4 - Set Admin password
SQL> @apxxepwd.sql password

Step 5 - Enable remote http connections
SQL> exec dbms_xdb.setListenerLocalAccess (l_access => FALSE);

Login
URL: http://127.0.0.1:8080/apex
Workspace: Internal
Username: Admin
Password: password