given traktors support for device targets it becomes a bit interesting to use one controller per deck. however with 4 decks .. you need 4 controllers. that being said if you have 2 identical controllers it becomes really use the write a single bome mapping that makes it possible to switch the virtual device depending on a single variable, thereby you can easily control 4 decks with 2 controllers and just having to map out a single deck in traktor.\
doing the same with a 2 deck controller is a lot more work, since you first need to remap one of the decks on the controller to essentially send the same midi messages before you can leverage the device target feature in the way described above.
Easiest and cheapest thing could be a BCR or a X-Session Pro.
But then again, the fanboys would laugh and tell you to opt for either a VCI-100 or a Allen & Heath XoneX
Also one to wait for, could be the new Native Instruments Richie Hawtin controller.