It’s important to start at the beginning, especially if you've never delved into four-deck mixing. Experiment with this most fundamental set up to get a feel for what it's like to handle four channels in the mix at the same time.
Most four-channel MIDI controllers let you access the transport controls of the two additional 'decks' via a shift function, which makes this an exercise in dexterity (and not to mention caution). A simple oversight and you can be pressing stop on the wrong deck, bringing the whole mix tumbling down.
If you practise straight mixing with four tracks on four decks you'll hone your controller skills. What’s more, you’ll probably soon realise how a quad of songs playing at the same time rarely sounds all that good.