Compute the probabilities of the Kth extension of a source for a given K?

A discrete-memoryless information source is described by the probability vector p = {0.2, 0.3, 0.1, 0.4}. I have to write a MATLAB file to compute the probabilities of the Kth extension of this source for a given K. So far I have written the following code:

K=2;

p=[0.2,0.3,0.1,0.4];

P=zeros(length(p)^(K-1),length(p));

Any idea how to continue?