how about 1 Otus and an additional xp10/xp5 ?
thats what I use and its nice and compact, controllers and soundcards in 1, the otus controlls decks a and b with the 2 layers, and u use the xp10/5 with the focus command in traktor for deck c and d 
the only dissadvantage i can se is that u always need an external mixer, but in almost all the clubs I play there is 1 alrdy so thats np, and at private/corporat partys u can bring your own mixer of choice,i have an vestax pmc-cx, not ideal for 4 channel mixin, but at least i have the possiblity to use 4 channels with the xtra subline, + the filters rock 