The DDJ-400 is not only a controller, but also has an integrated sound card (audio interface) and thus 1 chinch output for the main/master signal and 1 jack output for headphones. I suspect it will also be a bit bigger and heavier, but you can read that yourself from the respective product data sheets.

The DDJ-200 therefore has only one USB connection. The audio signal can be output via the normal headphone output in the PC, but I advise against this for professional use (quality/noise).
The audio signal (or several) can also be output via any interface supported by your OS. For example: NI Audio A8, NI Audio A6,... or integrated in the mixer like DJM-900NXS or even via a sound card integrated in the CD player like the CDJ-2000 which all deliver a very good sound.

Since almost all venues are now equipped with the more recent Pioneer CDJ and DJM devices, the DDJ-200 is also suitable for professional applications, but requires more knowledge and is therefore less plug and play capable.

I hope I was able to help...