In version 4.0, there is a new feature in correspondence that, when checked, will mail to the primary household member when available. This mailing is sending to the household, rather than the primary member, even though the box is checked.
We're currently evaluating this issue for a fix in a future patch or service pack.
Steps to Duplicate
Add 2 new constituents, mark them as spouses and put them in the same household
Go to manage correspondence and create a new correspondence process
For criteria say lookup id one of and use the 2 household individuals
For output format use Formatted mail correspondence output, mark the new ‘mail to primary household member when available’ option
Run the process and note it pulls in the primary member
Go to the primary member record and add him to a group as the non primary contact
Run the process again and note it pulls in the household
The expectation is that it would still pull in the primary HH member