horde/imp works with php3, not php4 ?!?

[Theodore Hope]

I've been running horde-1.2.4 + imp-2.2.4 on Apache/1.3.17 and PHP/3.0.18
(on BSD/OS 4.1) for some time and it's all fine.  
We're using postgresql 7.0.3 for horde's built-in phplib session stuff.

Now, I've upgraded to PHP 4.0.4pl1 (all else stays the same), and when I
try to log in to horde/imp it gives me the infamous "The document
contained no data" message.  I've double checked the "upgraded" php.ini
file, made sure all the php*_ directives have been changed to
php_value and php_flag accordingly.   Nothing strange shows up in any
of Apache's error logs.

Display_errors and Display_startup_errors are set to Off, and I've
set Log_errors On and set Error_log to a file in /tmp.  Nothing at
all is getting logged there.

I'm wondering if PHP4's built-in session facility (which is enabled)
is conflicting with Horde's Phplib session stuff.  Session.use_trans_sid
is set to 1.  However, Session.auto-start is set to Off.

(All this info comes from a phpinfo() that I'm calling from within 
the horde directory.)

Pls, any tips?  Thanks, 
 -T.H.