I am also running SMS 4.0.
I initially started with relays.ordb.org, but it blocked very few connections and we still received a good amount of spam.
I recently switched to sbl.spamhaus.org and it blocks many more connections, but unfortunately, I can't really see a reduction in the amount of spam that is sent our way.
relays.ordb blocked about 2000 connection attempts in 2 weeks. Compared to 14000 blockes by spamhaus over the same time period.