将sum应用到data.frame by substring分组，with R

Sample datas :

样本数据:

> mat1 = as.data.frame(matrix(c("D-J10-N1","D-J10-N2","D-J2-N1","D-J2-N2",3,6,5,7,8,4,2,3,4,1,2,3), ncol = 4));
> mat1
        V1 V2 V3 V4
1 D-J10-N1  3  8  4
2 D-J10-N2  6  4  1
3  D-J2-N1  5  2  2
4  D-J2-N2  7  3  3
> mat1 = as.data.fram