With `Prepend` we can add a line to all matrices in a set, like this:

    X = {{{1,3,7,6},{2,1,3,2},{3,1,7,2}},{{7,7,1,8},{9,3,4,2},{2,4,4,6,1}},{{1,6,7,1},{4,5,8,1},{5,5,7,3}}};
    Map[Prepend[{1, 1, 1, 1}]]@X

This code adds the line `{1,1,1,1}` to each matrix of the set `X`.

To concatenate a given matrix `2 x 4` with each matrix of the set `X` we can use `Prepend` twice. But if

    Y={{{2,6,8,2}, {9,4,9,8}},{{1,7 4,2},{7,6,9,6}}}

how can I obtain all the possible concatenations of the matrices from `Y` with those from `X` (in this order)?

The order is important. Each element of the resultant set is a matrix whose first two lines are a matrix from `Y` and the last ones is a matrix from `X`, like this one:

    {{2,6,8,2},{9,4,9,8},{1,3,7,6},{2,1,3,2},{3,1,7,2}}

The code have to work on `n x 9` matrices (`n < 5`).