[Exim] Incorrect Berkeley DB version

Just tried working with dbm lookups in exim for the first time. But
exim seems unable to determine the version of Berkeley DB that's
running.

I have db4 4.2.52 installed on OS X (using darwinports), but "exim -bV"
shows only "Probably Berkeley DB version 1.8x (native mode)". Didn't
think this was a problem until I tried a lookup directly from my
spamprobe db4 file using exim:

failed to open /.spamprobe/sp_words as a db (v1) file: Inappropriate
file type or format
                                            ^^^^

Any ideas on why exim can't detect the installed Berkeley DB version?
Where does exim look to get this info?

Brad