about four meters long,the room served as a ...中的about four meters long是定语还是补语？

当然是补语了
定语要修饰名词的
而句中用，分开了
句中about four meters long是served as 补语,补充说明作用

插入语有可能是。

补语

不给全句怎么看啊

我看是定语

没错的

是定语吧。
应该是这样的：

The room, (which was about four meters long),served as a ....

这里是简写了，而且前置了（放到主语前面）