As stated by Andy, IBM won't support it but here's how to get the 4.2.1
baclient to install

download 421 package into directory on 420 machine
cd into directory and type 'inutoc .'
edit the file called .toc in the directory to change the prereq statement to
4.2.0
smitty then installs it.

We're running it - I don't like it but the apps guys don't want to take the
risks (and time) involved in upgrading.

Not good but 4.2.0 was pretty bad as a release and, if my memory serves me
right, was only out for a short time.

Regards
Alan