Mohammed - just use randperm to generate four random numbers that you can use as indices into your array. Try the following
% generate the four random indices
randIdcs = randperm(length(A),4);
% initialize R to be the four numbers of A
R = A(randIdcs);
% remove those four numbers from A
A(randIdcs) = [];
If order is important, then just sort the output of randperm
randIdcs = sort(randperm(length(A),4));
