I also got a cheap 4mm diameter c-pen (no-name). Works well on the Tab S. It reacts on the slightest touch. And gives a bit more accuracy than the 6mm wide one.
Tested both on my Galaxy Note 1 for comparison. The smaller tip one has problems there to be recognized accurately. Needs certain amount of pressure. Also the glass surface on the Tab S is much smoother, giving a better sliding feel.