Well, almost all of you are right in one way or another.. ;-)

I have made some test today and here are the results:

I put in the 20m eyepiece looking on a nice external air conditioning
unit some hundreds of meters away. Looking down with one eye on the
eyepiece and the other to the floor, I marked where the projection of
the air condition on the floor started and where it ended. Now I had
35x.

Then I put the barlow with the same eyepiece and noted the projection
which was now almost double. Good thing. At least my Barlow was a 2x
one ;-) I marked the projection limits now at 70x.

Then I put the 11mm Televue eyepiece. The projection was somewhat
smaller which was ok at 64x. I put the barlow on and it normally
doubled. That was 128x.

At last I put the "4"mm eyepiece and guess what!!: the projection was
smaller at about 80% of the 128x one!!

So the "4mm" eyepiece is not a 4mm but something from 6mm to 7.5mm.
They have a wrong label on it saying "K 4mm".

Tomorrow I will do the same tests with greater accuracy to calculate
the exact mm of the eyepiece.

Thank you all for your help anyway!