Interesting. I seen to remember that at one point the reverse lookup for one of the 4.2.2.x addresses was "dont-steal-service-etc.somebody". I just send an ICMP ping when I need an IP to check a connection. I also seem to recall reading that the 4.2.2.x resolvers were multi-homed, but in both cases my memory could be faulty.
Level3's VP says they're public.
(They go up to 4.2.2.6 too, I think.)