Harry Potter and the Goblet of Fire Easter Egg - Sir Ian McKellen in Alastor Moody's Foe-Glass

From 0:50:34 to 0:50:38.
When Alastor Moody is giving advices to Harry Potter, in his office, Harry is looking at the Foe-Glass, then Moody says "That's a Foe-Glass. Lets me keep an eye on my ennemies".

Then we saw the Foe-Glass like Moody saw it.

For a very short time, you can see faces (ennemies of Moody, one of them is Ian McKellen's.
(Ian McKellen Gandalf in Lord og the Rings, Magneto in X-Men...).

Why ?
Because when Richard Harris (Dumbledore actor in HP 1 & 2) die, a rumours that Warner Bros ask him tto play Dumbledore. He don't say yes: "I have NOT agreed to succeed the late Richard Harris in the role of Dumbledore in the Harry Potter movies".
He has expressed interest in perhaps playing a different role some time in the future of the series.

I think it explain is apparition in HP 4.