Technics 1200-MK2; DJM-850; Maschine
Stop buying gear and learn to use what you have. Another piece of kit won't make you more creative. The things that matter are in you, not the gear.
Or, buy an F1 to use remix decks. It will take up one of your 4 decks (so you'll still be using the same max number) but will give some more creative options.
But, like everyone is saying, 4 deck mixing of normal tracks sounds like more than enough noise, and a lot to manage.