使用
<el-table-column label="长度(m)" align="center" prop="length" width="128">
        <template slot-scope="scope">
          <span>{{ scope.row.length | keepNumber }}</span>
        </template>
      </el-table-column>

注意，filters: 是单独的。不存放在methods：里

  filters: {
    keepNumber(value) { //过滤器保留4为小数
      const numM = Number(value).toFixed(5);
      if (numM === "NaN") {
        return "0.0000";
      }
      const realVal = numM.substring(0, numM.length - 1);
      return realVal;
    },
  },