Hi Aslam,
CUCM "nodes" (Pub/Sub's) must be on the exact same version. There is an option
during the install of the new node to upgrade to the proper version So you can
use the 6.1(2) discs and then patch to 6.1(4) during the new Sub install.
Here are the steps;
When you perform a fresh installation of Cisco Unified Communications Manager, you must define any subsequent servers (nodes) in the Cisco Unified Communications Manager Administration Server Configuration window before you can install Cisco Unified Communications Manager on each subsequent server. To define a subsequent node, click Add New, and perform the procedure in the "Configuring a Server" section. After you add the subsequent server, you can then install the Cisco Unified Communications Manager software on that server.
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/7_0_1/ccmcfg/b02servr.html#wp1020200
Installing Cisco Unified Communications Manager on a New Subscriber Server
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/install/7_0_1/cluster/clstr701.html#wp97669
Hope this helps!
Rob