i'm in the same boat ,sm4 is not realeased yet iirc , i was waiting for sm4 just cus it has knock control , but am thinking i can get a stand alone knock device for less than the cost difference between sm2 & sm4
don't know what other features there are ?
another point for me is mark will not have mapped one before although its probably well within his capability i don't fancy being guinea pig.
any cost info out for the sm4 yet?
so looks like sm2 for me