you actually want to do
for n=1:4
a{n}= randi(10,n)-1;
b{n}= randi(10,n)-1;
fprintf('n = %d:\n', n), A = a{n}, B = b{n},
end
as randi(9) is "Generate integer values from the uniform distribution on the set 1:9."
and what you have should be correct and does more than just 2 4x4 matrices. if you scroll up in the command window (as every calculation is displayed because there is no ';' at the end of the lines)
you should see that a and b is a cell array with varying size matrices in each cell
>> a
a =
[5] [2x2 double] [3x3 double] [4x4 double]
>> b
b =
[2] [2x2 double] [3x3 double] [4x4 double]
