Hi,
The way to do that is editing the view "product / quantity" and check the stock quantity. If the stock is under "5" change the text displayed.
Try to replace the line:
echo '<span class="hikashop_product_stock_count">'.JText::sprintf('X_ITEMS_IN_STOCK',$this->row->product_quantity).'</span><br/>';
By:
if($this->row->product_quantity<5)
echo '<span class="hikashop_product_stock_count">'.JText::_('Less than 5 products').'</span><br/>';
else
echo '<span class="hikashop_product_stock_count">'.JText::sprintf('X_ITEMS_IN_STOCK',$this->row->product_quantity).'</span><br/>';