How to create 4D mat, like dnn::blobFromImage do?

edit

do something like this:

Mat bgr = ... // you probably need to resize / crop it here

// preallocate the blob:
int siz[] = {1, 3, bgr.rows, bgr.cols};
Mat blob(4, siz, bgr.depth());

// get pointers to preallocated image planes 
vector<Mat> slices = {
    Mat(bgr.rows, bgr.cols, bgr.depth(), blob.ptr<uchar>(0,0)), // beware, hardcoded type here !
    Mat(bgr.rows, bgr.cols, bgr.depth(), blob.ptr<uchar>(0,1)),
    Mat(bgr.rows, bgr.cols, bgr.depth(), blob.ptr<uchar>(0,2)),
};

// split color planes
split(bgr, slices);