Hi Abbas,
From 4.2 there is a direct upgrade path to 6.x which is very good, but not to 5.x as you have noted;
Cisco CallManager Upgrade Paths
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_tech_note09186a008086cc7c.shtml
System Requirements
Make sure that you install and configure Cisco Unified Communications Manager Release 6.0(1x) on a Cisco Media Convergence Server (MCS).
You may also install Cisco Unified Communications Manager on a Cisco-approved HP server configuration or a Cisco-approved IBM server configuration.
Cisco Unified Communications Manager requires a minimum of the following items on the Cisco MCS.
2 GB of memory
72 GB disk drive
2 GHz processor
Supported Platforms
To find which servers support the Cisco Unified Communications Manager Release 6.0(1x), refer to the Cisco Unified Communications Manager Server Support Matrix at;
http://www.cisco.com/en/US/partner/products/hw/voiceapp/ps378/prod_brochure_list.html
and
http://www.cisco.com/en/US/customer/products/hw/voiceapp/ps378/prod_brochure0900aecd8062a4f9.html
From these CCM 6.0.(1a) Release notes
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/rel_notes/6_0_1/601acmrn_2.html#wp42710
Hope this helps!
Rob