net-mail/cyrus-imapd-2.1.13 misdetects db4 looking for db_create, while db4 provides db_create_4000 or db_create_4001
cyrus-imapd-2.1.14 is in portage which should now detect the proper versioned db4 symbols. Please test.
I think that this problem should really be fixed in cyrus, rather than ebuild.
Closing old bugs that were reopened in error.