Isabel - try the following
C = zeros(size(A,1),size(B,2)); % <--- (number or rows of A) x (number of columns of B)
for o=1:size(B,2) % <--- iterate over each column of B
C(:,o)=A*(B(:,o)); % <--- use : C(:,o) means to set all rows of oth column
end
Note that
C(o,size(B,2))=A*(B(:,o));
is equivalent to
C(o,5))=A*(B(:,o));
so the code is trying to assign a 4x1 column to a single element in C.
