Hi Bahman,
Cisco changed the Unity documentation policy at 4.0.5 You must use the appropriate 4.0.5 documentation (exchange,lotus notes,failover etc.) along with the attached 4.2 Addendum. That is why you can't find the 4.2 install guide.
Use the Installation Guide That Matches the Cisco Unity 4.0(5) or Later Configuration
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/prod_installation_guide09186a008042173f.html
Cisco Unity Documentation Addendum, Release 4.2
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_administration_guide_book09186a00805eb63a.html
Cisco Unity 4.2 System Requirements
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/prod_system_requirements_hardware09186a008061eb2a.html
Supported Hardware and Software, and Support Policies for Cisco Unity 4.2 and Later
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/prod_system_requirements_hardware09186a00806cd9d2.html
Hope this helps!
Rob
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