I believe there is a weight difference between 2 and 4 pc. rods. The 2 pc .will be a tad lighter and cast a bit differently.

Most stress type rod breaks occur at a ferrule, just above the female ferrule. This is due to the fact that a ferrule does not distribute the force equally but centers it at the convex point of the female ferrule. A ferrule must also resist twisting force which will loosen a ferrule. As the ferrule loosens, this become a "hinge" point and the upper section fails. This is why many spey rod caster actually tape their ferrules. Their casts loosen the ferrules and the rods are so long, it becomes a hassle to check them.

Fewer ferrule means fewer ferrule failures. I once broke both of my two 4 pc Sage fly rods at exactly the same place, just above the middle ferrule during a two day fishing trip. Just remember to check that the ferrules are tight. I have found that as the ferrule loosens, the rod gets less responsive.

http://www.springbrook.ca/images/TFOFLYRODWARRANTY.pdf

"Loose ferrules will also result in breakage. Regularly check the joints throughout the day to ensure they are tight. If they are constantly loosening, apply a very light rub of white candle wax to the male section...... To avoid having sections twist (when Spey Casting) and prevent breakage from loosened joints you should always tape the joints, black electricians tape is recommended."

http://americanangler.com/technique/busted

"5. UNSEATED FERRULES

Another easily avoided rod breaker is an unseated ferrule, which is like a time bomb, says Johnson: It's an almost surefire way to break a fly rod. 'The ferrule sections loosen up during casting, and if a fisherman does not reseat those sections, they will eventually break,' warns Jim West. 'It happens all the time.'

If you are casting weighted flies, sinking lines, or a Spey rod, it's imperative that you check your ferrules regularly. The constant stresses and twisting action of casting will inevitably loosen those fragile connections."

The top over tip ferrule and the spigot ferrule systems have decreased the stiff spot but have not eliminated them. If a caster is given a random 2 vs a 4 pc. rod to cast, most would not be able to tell. However, I do believe that if a fly fisher has use a 2 pc. rod for years and you blind fold him and have him cast his 2 pc. vs a 4 pc. version of his rod, that most would be able to tell. It is a question of muscle memory. I think there is a difference in how these rods cast but the difference is small and requires a practiced hand to tell the difference.

For practical reasons, I use mainly 4 pc. rods but I have older 2 pc. rods that I also use. Whether a 2 pc. or 4 pc. makes you a more effective fly fisher is debatable. In an actual fishing situation, I don't think it matters. The fly caster will compensate for the slight difference.