So you can just write a function with 4 16x16 matrix input and single 64x64 output.
HTML:
type matrix_16x16_t is array (0 to 15, 0 to 15) of integer;
type matrix_64x64_t is array (0 to 63, 0 to 63) of integer;
function big_matrix(A :matrix_16x16_t; B : matrix_16x16_t; C :matrix_16x16_t; D : matrix_16x16_t)
return matrix_64x64_t is
variable result : matrix_64x64_t;
begin
//TODO depands on how you wanna combine them A|B or other way
C|D
return result;
end function big_matrix;