As you're not getting many answers I'll suggest a low tech method that might get you start...
Use an ant exec task to fire off 4 background php processes writing their pid to a file which includes the build number (from environment presumably) to identify it.
Once build is complete run script again with a stop parameter and use the file naming system to find process ids, kill take and delete piddling files. Probably worth you having some sort of stale job cleaner in there too.
Shouldn't be too hard to knock up something that works until you can find a more elegant solution.
Edit:
Is this any good for you:
test.php: (this would be your worker script)
<?php while (true) { echo "Hello world" . PHP_EOL; sleep(5); }
runner.sh:
#!/usr/bin/bash
FILE_TO_RUN=test.php
if [ -z $TEST_RUNNERS ]; then
TEST_RUNNERS=4;
fi;
if [ -z $BUILD_NUMBER ]; then
echo "Can not run without a build number";
exit 1;
fi;
FILE="${BUILD_NUMBER}.run"
if [ -e $FILE ]; then
while read line;
do
echo "Killing process " $line
kill -9 $line
done
echo "Deleting PID file"
rm -f $FILE
exit 0
fi < $FILE
for ((i=1; i<=$TEST_RUNNERS; i++)); do
echo "Setting up test runner number " $i " of " $TEST_RUNNERS;
php $FILE_TO_RUN &
echo "PID number: " $!
echo $! >> "${BUILD_NUMBER}.run"
done
exit 0