If you had 2 fly rods one was a 2 section rod the other a 4 section both 9ft 4w same action. We are talking about weight, casting ability,sensitivity,durability,number of guides etc Which rod would be the better performer? Is there a difference because of the extra sections ?

If they are made of the same material, and identical in action, power, and weight. Then they will perform the same. In general, adding ferrules is going to require more material to reinforce the joints resulting in a slightly heavier rod, and each section will have a different taper than it may have in a blank of fewer sections. In the end, the added weight in a modern ferrule is not much. I would think in a blind test, if the blanks have the same action and power and differ in weight by a tenth of an ounce it may be tough to tell the difference depending on how the extra weight is distributed in the blank.

Stepping away from theory now. Even blanks that come from the same line with similar specs, but different numbers of sections are not always the same action and power, and it may be safe to say that generally they will differ a bit. The blank designer could be designing them for different applications and one may be more or less powerful and have a different action, but still fall within the same broader weight ranges and action classifications that the manufacturer uses to designate blanks.

Very true. Often the 2-piece and 4-piece editions were not designed around the exact same parameters to begin with. Very hard to compare for that reason. Although if all else is equal, the fewer pieces the better the performance should be. However, these days with such excellent ferrules, the difference is so slight as to be a non factor for most considering the added convenience of the multi-piece models, if you require it.

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I build nothing but fly rods and have not built a 2piece for about 6 years, that's 100s of rods. The convenience of multi piece for travel and storage far out weighs any of the very slight differences in the features you mention.