php4 and php5 together

Hi

I have the following urgend problem:

I need to use php4 AND php5 on the same apache2 installation on debian
sarge.
Until now, I had php5 running as module and no problems so far.

But now, I need to support ILIAS3, an php-based elearning plattform.
This does not support php5 so far.

What can I do? Is there a way to tell php5 to parse some files in
"compatibility mode"?
Or can I install php4 and php5 simultanously as modules?

If I install php5 as module and php4 as cgi, what will be the performance
implications?

Any hint would be very appreciated, the service needs to run until next
Friday.

Regards,
Steffen