Uncle Wang originally planned to process 72 parts in 9 hours, but actually processed 12 parts per hour. How many parts were actually processed per hour more than the original plan?
王叔叔原计划9小时加工72个零件，实际每小时加工12个零件，实际比原计划每小时多加工几个零件？
<gadget id="calculator">72 / 9</gadget>
<output>8</output>

<gadget id="calculator">12 - 8</gadget>
<output>4</output>

<result>4</result>