What is the difference between Ver. 4 and Ver. 8?

[Chris Zimmerman]

Please forgive my ignorance, but I have recently run into an issue with BIND
4.x.x (AIX 4.2.1), where I am more familiar with BIND 8.x.x.  My problem is
this:  we have an internal DNS server for hostname resolution running on AIX
4.2.1.  I have been trying to send e-mail between systems internally (UNIX
to UNIX, or NT to UNIX), but I have been unable as sendmail has been
complaining of a "Hostname Lookup Failure."  After conversing with the
sendmail folks for a few days, I decided to try running BIND 8.2.2 patch 5
on my local Linux machine, import enough DNS information to make it
functional for email between 2 systems, then try to send mail.  As soon as I
started BIND locally, even the queued messages from previous attempts were
delivered.  What I would like to know is what is different about 8 that is
preventing 4 from working?  

One thing that did arise during the troubleshooting was that the internal
DNS is giving non-authoritative answers for everything.  To see if it was a
config issue with the named.boot, I tried the script that converts the
named.boot to the named.conf, and then restarted my local DNS-everything
still worked.  We are looking to upgrade to BIND 8, but I will not only need
some solid reasons for doing so, but I need to see if I can fix the BIND 4
on the AIX machine until this upgrade can be completed.

Thank you,
Chris Zimmerman