Hi Paul,
the Mainstream support for CRM 4.0 ended 9 April 2013 (there is still the Extended support), this means (I don't remember if I read an official communication about this) that no rollups will be released for CRM 4.0
You can find the dates here:
http://support.microsoft.com/lifecycle/default.aspx?LN=en-us&x=14&y=5&c2=629
and also this post has some advises: (end of the article)
http://blogs.msdn.com/b/emeadcrmsupport/archive/2010/04/13/microsoft-crm-4-0-update-rollup-overview.aspx
Future Updates for CRM 4.0 (after Rollup 21)
Due to the low volume of fix requests for CRM 4.0, the CRM SE team will stop shipping CRM 4.0 rollups on a regular schedule.
CRM SE will service CRM 4.0 requests on an individual basis using our Critical On-Demand process.
CRM SE will continue to evaluate the need for Update Rollups for the remainder of CRM 4.0’s supported lifecycle.
If your system must migrate to SQL 2012, I suggest to clone your actual environment and try the update and test if the issue will be present in your system.
Hope it helps
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