I had studied in xxxx for four years文法有沒有錯呢?

"I had studied in xxxx for four years" is not grammatically correct

"I have studied in England for four years".
= Studied for 4 years and is still studying in England.

"I studied in England for four years".
= I don't study there now

One of the applications of "Had + ppt" is as follows:
"I had studied in England for four year when you came to England".
=Both of them were past actions. The (had+ppt) event began earlier.

1. ... study in xxx (a place) 指的是" 在(某處)讀書 ", 而並非" 讀課程 "。如果你想說" 完成一個課程 ", 可以說:

to complete a course

完成一個四年課程: to complete a four-year course

" had/have studied in xxxx for four years " 的意思是 " 已經在 xxxx 讀了四年 "。這句並沒有說明你已經完成課程，只講出" 你於(某處) 讀了四年 " 而已。

" had " 與 " have " 的分別是﹕前者用於 reported speech，後者屬 direct speech。

所以，如果你想表示" 已經完成一個四年的課程 ", 可以說﹕

I have completed a four-year xxxx course.
OR
I have completed my four-year xxxx studies.

希望對你有幫助。

請問你呢句 sentence 是在那些場合寫的? 如果是講一個故事的話，你這個sentence 就對了，但如果是在普通情況下，你就應該用 have 代替 had。
例: What course have you studied? I have studied in xxxx for four years.