Hi,
Which version of Oracle you are using.If you are using Oracle 8i and above you can write database or schema level triggers to restrict users to connect the Z,Y,Q users or else dont give the passwords of Z,Y,Q to user A,B,C,D.
Oracle8i Enterprise Edition Release 8.1.7.0.0 - Production
PL/SQL Release 8.1.7.0.0 - Production
CORE 8.1.7.0.0 Production
TNS for 32-bit Windows: Version 8.1.7.0.0 - Production
NLSRTL Version 3.4.1.0.0 - Production
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