There's only one postgresql version, only one php script (and then, only one base/table for the test).
Both versions of php (and apache) are running at the same time.

The sample program:
<?
for ($j=0 ; $j < 10 ; $j++)
{
  echo "open connection $j<br>";
  $cn = pg_connect("host=xxxx port=5432 user=xxx password=xxxx dbname=xxxx");

  $sql = "Select * from os_log limit 10;";
  $rs = pg_exec($cn, $sql);

  if ($rs == false)
    echo 'bug<br>\n';
  else
    echo 'ok<br>\n';
  flush();
}
?>

----------------

php 4.0.2 result:

open connection 0
ok
open connection 1
ok
open connection 2
ok
open connection 3
ok
open connection 4
ok
open connection 5
ok
open connection 6
ok
open connection 7
ok
open connection 8
ok
open connection 9
ok
----------------
php 4.0.4pl1 result:
open connection 0
ok
open connection 1
ok
open connection 2
ok
open connection 3

Warning: 1 is not a valid PostgreSQL link resource in /opt/web/netos.dev/db.php on line 7
bug
open connection 4
ok
open connection 5
ok
open connection 6
ok
open connection 7

Warning: 5 is not a valid PostgreSQL link resource in /opt/web/netos.dev/db.php on line 7
bug
open connection 8
ok
open connection 9
ok

----------------

php compilation options:

for 4.0.4pl1:

./configure --prefix=/opt/php-4.0.4pl1.test --with-apache=../apache_1.3.12 \
--with-config-file-path=/opt/php-4.0.4pl1.test/lib \
--enable-track-vars --enable-ftp --with-readline --with-imap=/opt/c-client \
--with-mysql=/opt/mysql --with-pgsql=/opt/postgresql \
--enable-trans-sid \
--enable-sysvsem --enable-sysvshm --enable-memory-limit \
--enable-debug=no --with-ttf=/opt/freetype --with-t1lib=/opt/t1lib \
--with-regex=php --with-xml \
--with-jpeg-dir=/opt/jpeg --with-tiff-dir=/opt/tiff --with-png-dir=/opt/libpng \
--with-zlib-dir=/opt/zlib --with-gd=/opt/gd \
--enable-sockets  \
--with-swf=/opt/libswf --with-sablot \
--with-pdflib=/opt/pdflib --with-dom=/opt/libxml2 --enable-bcmath

php4.0.2:
There's not --with-dom due to configuration/compilation  problems, but it isn't directly linked with postgresql:

./configure --prefix=/opt/php-4.0.2.test2 --with-apache=../apache_1.3.12 \
--with-config-file-path=/opt/php-4.0.2.test2/lib \
--enable-track-vars --enable-ftp --with-readline --with-imap=/opt/c-client \
--with-mysql=/opt/mysql --with-pgsql=/opt/postgresql \
--enable-trans-sid \
--enable-sysvsem --enable-sysvshm --enable-memory-limit \
--enable-debug=no --with-ttf=/opt/freetype --with-t1lib=/opt/t1lib \
--with-regex=php --with-xml \
--with-jpeg-dir=/opt/jpeg --with-tiff-dir=/opt/tiff --with-png-dir=/opt/libpng \
--with-zlib-dir=/opt/zlib --with-gd=/opt/gd \
--enable-sockets \
--with-swf=/opt/libswf --with-sablot \
--with-pdflib=/opt/pdflib  --enable-bcmath