Hi all,

I'm looking for a cutter to cut notches in aluminium, the aluminium will be
4 mm thick, and the width of the notch must be a narrow as possible.
I'm looking for a brand or manufacturer of such a thin cutter which is able to
do this.

Try Sandvik Coromant.
http://www.coromant.sandvik.com/us

Have you tried Harvey Tool? They have some really small cutters.

http://www.harveytool.com/index.php