Case 4: All redo log file or any one redo log file missing and we have no backup copy of redo log file or no multiplexing redo log file.

If all or some redo log is missing. Here's an example, where I attempt to startup from SQLPlus when a redo log is missing:

SQL> startup

ORACLE instance started.
Total System Global Area 122755896 bytes
Fixed Size 453432 bytes
Variable Size 67108864 bytes
Database Buffers 54525952 bytes
Redo Buffers 667648 bytes
Database mounted.

ORA-00313: open failed for members of log group 1 of thread 1
ORA-00312: online log 1 thread 1: 'C:\O\ORADATA\REDO01.LOG'

SQL> recover database until cancel;
Media recovery complete.

SQL> alter database open resetlogs;
Database altered.

That's it - the database is open for use.