For the disconcerting DJ, there is no need to spend bucket loads on multi-channel mixers when you're likely to never (or rarely) use it to its full potential. The DJM-400 prevents this from happening as, after a long time coming, Pioneer have produced a modern unit that boasts only what you need and nothing else - all to the quality of its bigger brothers. With more Sound Effect choice than even the classic DJM-500 and brighter LED lights that are far clearer when viewing the unit from an angle, there is little to dismiss if you're space saving.
Perhaps the only quibble is the price, something that isn't really surprising since Pioneer have recently been in the mood of jacking up RRP of most of their music equipment. On the basis of comparison, there are bigger and more functionally impressive mixers out there for the price, but by looking into these you would missing the point. The DJM-400 is designed to be compact and of high quality, which is where the money goes.
Need the extra 2 sound channels? Check out the DJM-500 which is now stupidly cheap.