我们需要编写一个函数，实现将阿拉伯数字转换为英文数字的功能。下面给出一个示例代码：

def number_to_words(num):
    """
    将阿拉伯数字转换为英文数字
    """
    # 定义数字和其英文表达的对应关系
    ones = ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine']
    tens = ['', 'ten', 'twenty', 'thirty', 'forty', 'fifty', 'sixty', 'seventy', 'eighty', 'ninety']
    teens = ['ten', 'eleven', 'twelve', 'thirteen', 'fourteen', 'fifteen', 'sixteen', 'seventeen', 'eighteen', 'nineteen']
    
    if num < 0 or num > 9999:
        return 'number out of range'
    
    if num < 10:
        return ones[num]
    
    if num < 20:
        return teens[num - 10]
    
    if num < 100:
        return tens[num // 10] + ('' if num % 10 == 0 else '-' + ones[num % 10])
    
    if num < 1000:
        return ones[num // 100] + ' hundred' + ('' if num % 100 == 0 else ' and ' + number_to_words(num % 100))
    
    if num < 10000:
        return ones[num // 1000] + ' thousand' + ('' if num % 1000 == 0 else ' ' + number_to_words(num % 1000))

该函数使用递归思想实现了将4位及以下数字转换为英文数字的功能。下面是一些示例：

num = 145
print(number_to_words(num))
# 输出：one hundred and forty-five

num = 3000
print(number_to_words(num))
# 输出：three thousand

num = 9999
print(number_to_words(num))