I think that that that that that student wrote on the blackboard was wrong.我认为那个学生写在黑板上第四个that引导的是定语从句,还是同位语从句呀?（貌似第四个that在从句中不充当成分呀）

I think that-1 that-2 that-3 that-4 that-5 student wrote on the blackboard was wrong.
为了看的方便,把5个that都依次编上号,下面进行拆
1. I think (that-1) that-2 that-3 was wrong （that-1,连接词,后接宾语从句,自身无意义,可省略；that-2,指示代词,“那个”；that-3,指的是写在黑板上的单词“that”）
2. that-2 that-3 that-4 that-5 students wrote on the blackboard was wrong （that-4,连接代词,引导定语从句,代替写在黑板上的单词'that'做定语从句（that student wrote on the blackboard）中wrote的宾语）
3. that-5 student wrote on the blackboard （that-5,指示代词,“那个”学生）
全句意为：我认为那个学生在黑板上写的那个单词（that）是错误的.
这个句子充满娱乐精神,但没有用武之地,于交流无益；如果想表达相同语意,可以改写为：
I think the that which the student wrote on the blackboard was wrong.
限定词the 代替 指示代词 that
连接代词which 代替 连接代词 that
宾语从句连接词that 可省略