Comparing displacement, a 30-50% increase will give comparable horsepower. So a .56-.60 4s is comparable to a hot .40 2s, a .70 4s is comparable to a .46-.50 2s, etc. Which 4s you're talking about makes a significant difference too. Saitos and the newest OS's make the most horsepower for their size, so you don't quite have to go as big with them as you do with older OS's or one of the other brands. The weight will be close to the same too since 4s mufflers are much smaller than 2s mufflers are. All that said, it's not an apples to apples comparison. If you match horsepower, the peak will be at a much lower RPM so the plane will respond to the throttle differently. The 4s will pull the plane through maneuvers better and accelerate it better, while the 2s will generally make more overall speed.
To give you a real world impression, I just went from a .46 2s to a Magnum .52 4s in my Piper Cub. There is a definite drop in overall power on the top end, evidence by less top speed. But the throttle response is much more linear and it still pulls though big loops and hammerheads with authority. I like the feel better with the 4s, but I do have to manage the speed a little better and generally find myself flying at a higher throttle setting that I used to with a 2s engine.