I have db4 installed.  My assumption, though, was that there was some 
  reason to use db2 rather than db4 for this package, and I'd like to 
  figure out the proper way to do that.  Perhaps set _BDB_TYPE in the 
  cucipop Makefile?

Variables with _ are internal and should not be set.  Plus, you would
be overriding logic in the bdb.buildlink3.mk file, and that seems to
be asking for trouble.

In bdb.buildlink3.mk, it seems like there is intent to have only one
of db[234] installed and used, but they don't seem to conflict, and
the comments don't explain what is supposed to happen and why when a
package lists db2 as the only acceptable version and db4 is the
default.

cucipop's Makefile should briefly explain the reason for the db2
requirement.  If that's really true and reasonable, then it would seem
that there could be packages that really require each of db2/db3/db4,
and this conflicts with bdb.buildlink3.mk's apparent requirement to
use only one.  But, the BDB framework has changed since that was added
in 1.14 in 9/2004, so it could just be a historical artifact.

You might check out dovecot; in addition to bdb grief cucipop has a
non-free license.

-- 
        Greg Troxel <gdt@ir.bbn.com>