//写法1:
$data = array();
$data['id'] = array('in','4');
$tag = $tag->where($data)->select();
//写法2:
$tag = $tag->where(array('id'=>array('in','4')))->select();
$tag = $tag->where(array('id'=>array('in',array('4'))))->select();not in和in方法一样,in替换成not in即可