Probably, you can find the php4-cgi package with the command
apt-cache search php4-cgi
If you've found it, you can install it with apt-get.
Although it is written for Sarge, I think you can have a look at Falko's howto, which is published here:
If you've installed php4-cgi as well, you have to tell ISPConfig when it is needed to use php4 instead of php5.
You can do that within the Apache directives field of the website which needs php4.
(I guess ISPConfig will use php5 as default, since your upgrade to Etch).
If not, you will add the directives for php5 as well, when needed.
I hope this will help to put you on the right track.