是的，一般说来，一个动词前面最多可以有四个助动词，网友的句子理论上是对的，但总的说来，这类用法比较少见：
Jim must have been being told while I was outside. 我在外面的时候一定有人在告诉吉姆。
但更通常的说法应该是：
Someone must have been telling Jim while I was outside.
当然，有时因特定语境的需要，人们也会选用这种表达：
The cake must have been being baked. 当时蛋糕一定正在烤着。
