Nope. The issue is that you need a certain airspeed through the pipe to keep the dust suspended, and the required speed goes up on vertical runs. So if you make the pipe too wide, the dust will literally fall to the bottom of the duct and build up, giving unpredictable performance.
Now it's true that a 6" duct that narrows down to 4" for a short segment will be better than all 4", but only if the speed in the 6" duct is sufficient to keep the dust suspended. So your best bet is to modify tool hoods if necessary to get 6" ports on the tool. (Or multiple smaller ports, like two 4" ports or a 5" and a 3".) If you're stuck with a 4" port, you might want to open up another port near that tool to ensure there is enough airflow (in CFM) in the 6" duct to get the necessary airspeed (measured in feet/min).
One last thing...it's not enough to just put a 6" port on the tool, you have to make sure that there is enough opening on the tool for intake air to supply the air that's being sucked out. For example, don't go sealing up the tablesaw cabinet tightly, you need a certain amount of gap space for that make-up air to flow in.