Cyclic cron tasks
0  8  *  *  1/4  echo "Variation A" | mail -s "Test email" admin@mydomain.com
0  8  *  *  1/4  echo "Variation B" | mail -s "Test email" admin@mydomain.com
0  8  *  *  1/4  echo "Variation C" | mail -s "Test email" admin@mydomain.com
0  8  *  *  1/4  echo "Variation D" | mail -s "Test email" admin@mydomain.com

0 8 * * 0 somescript.sh

#!/bin/sh

STATEFILE='emailstate'
VARIATION=`cat emailstate 2>/dev/null`

case "$VARIATION" in
    '4' )
        # send variation 4
        echo 'Sending variation 4'

        VARIATION='1'
        ;;

    '3' )
        # send variation 3
        echo 'Sending variation 3'

        VARIATION='4'
        ;;

    '2' )
        # send variation 2
        echo 'Sending variation 2'

        VARIATION='3'
        ;;

    *   )
        # send variation 1
        echo 'Sending variation 1'

        VARIATION='2'
        ;;
esac

echo $VARIATION > $STATEFILE