Well, after careful calculations here are the results:

The Celestron Ultima Barlow magnifies by a factor of 2.2x with all
eyepieces.

And, of course, the "4mm" eyepiece is not a 4mm but a 6.5mm one,
giving 108x on my scope.

I guess it is still a kellner. It wouldn't bother me if it was a
Radian though...