3 inch is a tad less then 4" but not by much, but I still need the size drop to develop the velocity that I want at the pump head that and eas of routing it.
Look.
You don't develop velocity by restricting the pipe. The velocity you get is directly proportional to the dynamic pressure just behind the opening, REGARDLESS of the size of the opening. The trick is to keep that pressure as high as possible at the point where the water leaves the pipe. (Yes the water must move faster in the narrower pipe than in the wider pipe feeding it. But that happens because the narrowing of the pipe produces a back pressure that slows the water in the wider pipe, to get the acceleration as it transitions to the narrow one. You'd have gotten higher flow rate out the final opening if you'd run the fat pipe the full distance.)
The max you'll get is the weight per cubic inch of water times the number of inches of head, but you derate that by the pressure drop from friction with the pipe. The narrower the pipe the more drop per unit distance at a given flow speed (due to smaller average distance from the wall to any given slug of water) and even moreso the more drop per unit distance at any given flow in galons per minute.
To maximize the flow from an opening of a given size, make the pipe as fat as possible ALL THE WAY FROM THE SOURCE TO THE OPENING. Then bring it down to the opening size just as it approaches the opening.
If the pipe were totally frictionless , the opening pointed upward, and there was no wind resistance, the jet of water would emerge at a speed that would take it up to the height of the surface of the water at the source, regardless of the size of the opening. Another way to say it: If the pipe were frictionless the water would emerge from the opening (of any size) at the speed it would achieve in a no-air-friction fall from the surface height of the water at the source.
But pipe ISN'T frictionless, and narrower pipe drops pressure more per unit length than wider pipe.
So if you can afford four inch rather than three inch pipe (and you have more than about a quarter-inch opening at the turbine end) you should go with four inch. The difference between three and four inch pipe, when it comes to friction and pressure drop, is very large.