在我小学四年级的时候用英语怎么说

在小学四年级的时候用英语可以说"in fourth grade of elementary school"。其中，"in"表示在某一时间或地点，"fourth"表示第四个，"grade"表示年级，"elementary school"表示小学。

例句：

I was in fourth grade of elementary school when I learned to play the piano.
(我在小学四年级学会弹钢琴)
He was in fourth grade of elementary school when his parents got divorced.
(他在小学四年级的时候他的父母离婚了)