10 years ago 4 piece rods were not very common and 2 piece rods were the norm. the reason being that the 4 piece rods lacked in feel and sensitivity of the 2 piece.
now days, as technology has advanced, most rods are 4 piece and for a good reason. the fit of todays 4 piece rods is way beyond the fit of the ones from a decade or so ago, not so these days.
another big reason is traveling. nobody wants to pay 25 bucks or whatever stupid cost to put their 2" diameter fly rod tube on a plane
so in a nutshell- if your traveling or are cramped for space-go 4 piece. it wont effect the action of the rod at all