You really have my wheels turning with this one because I am Rose crazy and wasn't even planning to try the kayfun4. Hmm...
I love my Erls but caved and ordered a KF4 yesterday myself.
I was never really a fan of previous iterations of the Kafun because of the restrictive air flow even with the afc screw removed. From what I understand, the KF4s air flow can be opened up wider than the earlier versions.
I tried to get a 4S kit but couldn't find any in stock anywhere.