Yes it is a lack of ammount of controls thing mainly - if i were to implement deck C and D at this time it would be taking the place of deck a and b when i engage them which is a concept i am not terribly fond of. That means i would have to use alot of soft-takeover which i peronally dont like.
Ill upgrade to 4 decks when i have the hardware that lets me use my controllerism techniques on 4 decks comfortably all at the same time.
Currently i use a Vestax VCI-100, a Stanton scs 3d aka dascratch and an arcade button controller made by midifidler to control 2 traktor decks + two virtual looping decks in ableton, personally i think this is pushing the hardware as it is without adding deck c and d.
My style is in the on the fly rearangments of tracks as opposed to a richie hawtin style layering of tracks - you have to remember i mix predominantly drum and bass, so the layering method that works well for minimal doesnt doesnt work as well on this sort of music.
My setup revolves around using every kind of digital beat juggling technique under the sun - cue’s jump, beatjumps, live looping, beatmashing and soon ill be implementing clists reaktor beatlookup as opposed to laying lots of material. When i have the hardware where can use all of those techniques, all at the same time on 4 decks then ill change to a 4 deck solution.
That all that said currently i have 2xtraktors decks and 2x Ableton channels so i do have plenty of layering power when it is needed - i can load a song, get a loop from a song going in ableton which frees that deck up to grab another part of the song or to load up another track up into the deck.
Using a mixture of Clists beatlookup and just simple clip retriggering in Ableton i will be able to remanipulate the live loop again 
I am quite capable of making my hardware control another 2 decks in traktor ontop of what i am already doing but those sorts of preset switches(bomes lingo) aka modifiers(traktor lingo) ontop of what i already have just wouldnt feel right for me at all - it would be an excessive use of soft-takeover which would end up confusing the hell out of me as every time i switched to deck C or D half of the faders/knobs would be in the wrong position.
The use of more rotaries might seem to be a smarter option than the absolute knobs/faders on the vci, but however i strongly rotaries lack the haptic feedback needed to perform controllerism.
Ill go probably go 4 decks and 2 more channels in Ableton when better suited hardware comes out or failing that when i build my own rig from scratch.