four decks, nothing hidden
Every four-deck controller I've used banks two decks onto one set of controls. You hit a button, the knobs change meaning, and half your rig goes invisible. In a studio that's fine. In a dark booth at 1am it's a small tax on every move you make.
So I specced the opposite: a surface where every control for every deck is permanently live. No deck-switch button, no shift layers, no menu diving.
the shape
It sits on a decksaver, on top of a CDJ you aren't using. 320 by 415mm, the CDJ-3000 footprint exactly, about 45mm tall. Four identical vertical strips: library across the top, then per-deck transport and loops, then dual FX, then the mixer section reading top to bottom the way a channel strip should.
40 potentiometers, 5 encoders, 38 buttons, 4 toggle switches. Around $290 in parts.
the part I like most
Each channel gets a toggle that flips its three EQ knobs between normal 3-band tone control and stem isolation — vocals, drums, bass.
The knobs don't move. The firmware just sends different CC numbers depending on which way the switch is thrown. Your software learns both sets once, and one physical switch swaps a whole channel strip between two completely different jobs. It's the cheapest thing in the build and probably the most useful.
what it doesn't do
No audio. It's a control surface only, with the interface as a separate box on the same powered hub, so if one fails you swap that one thing instead of the whole rig.
And it won't replace a jog wheel for scratching. The per-deck encoders are for scrubbing and needle-search. If you scratch, this isn't your controller.
where it actually is
This is a design, not a finished object. It's specced to the point where the next step is ordering parts, and I'm publishing it in exactly that state, gaps and all, because build references written afterwards quietly leave out all the decisions that were hard at the time.
The slowest phase, for what it's worth, is the MIDI mapping, and it doesn't need the board to exist. That's what the prototyping piece is about.
Full reference — layout, control inventory, the Teensy brain and its iOS gotcha, multiplexing 40 pots onto three pins, the button matrix, MIDI map, bill of materials and build order — is in the Lab: 4-Deck MIDI Controller Build Reference.