A few questions about the DDJ-RX

My honest opinion: You do not need this yet if you ask this way.
You can easily use the 4 channels with the controller only, as it has a 4 channel mixer and switching the decks is very easy.
When you are able to fully 4 channel mix songs with just the controller, then think about adding something. Currently by what you write alone I honestly do not think you are ready to add something to the RX. Most controller DJs even then do not bother to add decks and can 4-channel mix perfectly.

When you can do this ask again: