The big matrix X is not necessary non-singular, because we can find a big matrix with four sub-matrices are non-singular but the matrix X has two parallel rows are identical entries which det(X) = 0.
Thank you very much, in my work M1 not equal M3 and M2 not equal M4. All the matrices are different, square , tri-diagonal, and non-singular. and also i am sure this matrix is non-singular, but i need to know how to prove that ?