most servers have php 4 in the path. i run my cron jobs in php 5 by specifying the path to php 5, which is /usr/local/php5/bin/php. so an example job that runs at 3:30 am every day looks like this:
30 3 * * * /usr/local/php5/bin/php -f /home/username/cron/job.php
i'm not sure how you would set the path for a cron job as sXi mentions, but that might work too.
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