You can concatenate the matrices like this
a = [1 2; 3 4];
b = [4 5; 7 8];
c = [9 10; 11 12];
d = [13 14; 15 16];
Q = [a b; c d]; % this matrix is 4x4
To do operation on a block of Q, you can use indexing
Q(1:3, 1:3) = Q(1:3, 1:3)*2; % multiply top-left 3x3 block with 2
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