Hi Marc/Chris,

Here is the info I found for this question. There is not much for CCM 4.3 :) (This would indicate no to 5.x, Yes to 6.x)

Cisco Unified CallManager Release 4.3(1) is based on release 4.2(3) refer to the Cisco Unified CallManager Documentation Guide for a list of documents that are related to Cisco Unified CallManager release 4.2 at the following URL:

http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_documentation_roadmaps_list.html

From this doc;

http://www.cisco.com/en/US/products/sw/voicesw/ps556/prod_release_note09186a00807d935c.html#wp1012707

Existing customers using Cisco Unified CallManager 4.2 will be able to upgrade to the Cisco Unified Communications Manager 6.0 release.

From this doc;

http://www.cisco.com/en/US/products/sw/voicesw/ps556/product_solution_overview0900aecd805e19e7.html

Note, this thread with an excellent answer from Jorge @ Cisco;

CLIP "Correct, neither CCM 4.2(1) nor CCM 4.2(3) are able to upgrade to CCM 5. But they will be able to upgrade to CCM 6."

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Unified%20Communications%20and%20Video&topic=IP%20Telephony&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.1dddef86/0#selected_message

CCM 4.2 (CCM 4.3) to CCM 5.0/5.1 is not currently supported. Have a look at this excerpt from an excellent upgrade doc;

Upgrade from Cisco Unified CallManager 4.2 to Cisco Unified CallManager 5.0

This upgrade is currently not supported.

From this doc;http://www.cisco.com/en/US/products/hw/voiceapp/ps378/prod_brochure09186a0080174928.html

Hope this helps!

Rob