There is a division formula, the dividend is 219, Xiao Ma mistakenly calculates the dividend as 291, the resulting quotient increases by 4, and the remainder remains unchanged, so what is the divisor?
有一个除法算式，被除数是219，小马错将被除数当作291来计算，结果商增加4，余数没有变，则除数是多少．
<gadget id="calculator">291 - 219</gadget>
<output>72</output>

<gadget id="calculator">72 / 4</gadget>
<output>18</output>

<result>18</result>