oracle 12c rac switch from mounted to open
Connect to third instance and issue 

SQL&gt; alter database open;


then to fourth instance and do the same.

But if you want to open all instances at once, you need to use srvctl utility:

srvctl start database -db db_name


https://docs.oracle.com/database/121/RACAD/GUID-C9DB4DCB-34B9-42F0-8AB4-33199087CE98.htm#RACAD900