Official comment
Welcome and congrats on the purchase of the DDJ-400!
Given that the DDJ-400 is a brand-new product, that's natural. Certainly, many of the other DDJ videos posted by Pioneer DJ and others will apply to learning the product, especially those of its big brother, the DDJ-1000.
Regarding the specifics of the DDJ-400, read the manual, watch any videos or reviews you can, then play. Honestly, the learning curve isn't terribly steep as the majority of the features are within rekordbox itself (and there's tons of videos for that).
Please let us know if you have any specific questions.