Every time you ad a ferrul, you ad labour...I would be surprised if 4 piece rods are the same price as 2 piece UNLESS they are mass manufactured. I got a tour of the Lami factory and believe me when I say making a blank is labour intensive project!
Funtionally, I think a 4 piece would feel differeent than a 2. Think of it, at the ferrul, there is 2 times the material in the blank and most ferruls are wrapped and epoxied to finish the rod. This must ad weight and more material (I would guess) would also change the action slightly...enough to be noticable as you cast...can't comment on that for sure.