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).

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}},{{1,6,7,1},{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).