I have no clue where this "4%" comes from. The basic models on CO2 and the greenhouse effect are quite well understood, though the elaborated models are, in my view, much more speculative. They could be, in fact, they definitely are "wrong", but they could be wrong in either direction. "All models are wrong, some models are useful."
The increase from 280 ppm to 410+ ppm is nearly all due to man's actions. That looks like a significant increase to me.