having just made the switch back to 2 piece bars I have to say the only difference I notice (apart from some of the geo) is the actual stiffness of 4 piece bars. They just feel very solid and don't flex much... I always ran them for the geometry because no companies made 2 piece bars with 4-5 degree up-sweep. Now I am running Tree Moto Bars in a 9.5 rise with 4 degree up-sweep and they are more "flexy" than my old big 4s but I am still kicking myself cause Stranger is dropping the Gabe Brooks bars in the same geo as my tree bars (i think they may have 5 degree up-sweep which is awesome)... However I have come to really like the look and feel of my tree bars as my wrists suck now and the flex helps a bit
I think one key to a stiff 2 piece bar is the cross bar. I've noticed all the 2 piece bars I have with fat ass crossbars (Sunday Broadcaster, Oddy Gary Young) feel like they rival 4pc bars for stiffness and they all have 7/8" crossbars. Which makes sense since the way 4pc bars are usually made, they end up having 7/8" bars by design.