For the roman number system digits greater 4000 are supported. In reality these digits are shown with a crossbeam on top of the digit. As the crossbeam can not be shown within the computer, an underline has to be used instead of it.
<?php
$great = '_X';
$locale = new Zend_Locale('en');
$unit = new Zend_Measure_Number($great,Zend_Measure_Number::ROMAN, $locale);
// convert to the decimal system
echo $unit->convertTo(Zend_Measure_Number::DECIMAL);