阅读下面文段回答问题。

        I'm Mrs Zhou. I'm a teacher in No. 1 Middle School(第一中学). This is Li Lei. He's Chinese. He is in Class 2, Grade 7. He's my student, and my son(儿子)too. He's twelve.

       That girl is Jean. She's American. She's eleven. And that boy is Jack. He's English. He's ten. They're in Li Lei's class.

I'm their Chinese teacher. All the students and the teachers have two names: an English name and a Chinese name. Li Lei's English name is Tom. My English name is Mary. Their English teacher Miss Jin's English name is Sue. Jean's and Jack's Chinese names are Wei Hua and Zhang Li.

已知一组正数$x_1,x_2,x_3,x_4$的方差为$S^2=\frac{1}{4}\left(x_1^2+x_2^2+x_3^2+x_4^2-16\right)$，则数据$x_1+2,x_2+2,x_3+2,x_4+2$的平均数为（    ）