<select name="userType" id="userType" onchange='javascript:if(this.selectedIndex=="3") areaShopId.style.display="none"'>
<option value="0">0</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3" selected>3</option>
</select>
<option value="0">0</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3" selected>3</option>
</select>
'javascript:if(this.selectedIndex=="3") areaShopId.style.display="none"
再在下拉框的onchange事件中也加入
'javascript:if(this.selectedIndex=="3") areaShopId.style.display="none"