Is it possible to calculate the maximum flow in undirected graphs?

Hello,

I have constructed an undirected graph G and I want to calculate the maximum flow of it. I tried this:

G = graph(true(5000), 'omitselfloops'); 
p = randperm(numedges(G), 10000);
G = graph(G.Edges(p, :));
G.Edges.Weights = randi([1,200], 10000,1);
mf = maxflow(G,4000,666);

If I choose any (4000, t) node pair, i.e. (1<t<5000), will get the same value, which is "3". I don't know why!

How can I also determine the source and sink nodes in undirected graphs?