photography : Eksovichea Tito Hak
...4 deck implementation over 2 Blackberry Playbooks with the Pacemaker app?

In theory if they sorted it on the Pacemaker software front to allow for the linking of 2 Blackberry Playbooks via USB cable this could work.

doing it Without an external mixer (would need software modification)...

For a four deck setup without need for an external mixer you'd have to have a mirrored music collection/"record box" to each of the two playbooks so that you could actually directly record and playback the full set as it was performed later to each of the single Blackberry playbook devices. Because they both have the full library from which the set was put together they'd both be able to hold a complete recording of the set for reviewing and editing later.

Generally I'd prefer to work completely in Ableton Live if I using a laptop with a 4 channel mixer since there's already enough to be occupied with when trying to dynamically mix and remix live on the fly and off the cuff across 4 tracks in real time even without the "once learned to bike ride" chore of beat matching, but if the vanity of not appearing to be checking my email was a big deal to me and I wanted to experiment with futuristic centric DJ gear using multi-touch on hardware designed to work reliably for specialized touch based interface DJing I'd try this...

With a full sized external analog/digital 4 channel mixer that you already own (...a project for under £200 that doesn't necessarily need software modification for the tablets, vastly cheaper, vastly more portable then a single £2500 PIONEER NEXUS CDJ turntable and vastly more cutting edge with actual real world application potential)...

This would be even easier to set up, but obviously you couldn't record directly on the actual blackberry device's for obvious reasons. However you could possibly keep the (imaginary and none existent) USB cable link idea from the the last setup to keep the two devices in data playback sync and to optionally allow for easier syncing during performance mixing. But even without an imaginary potential syncing means between the two playbooks it would still be a usable setup. Everything else like EQing controls would obviously be done via the external mixer and set recording on an external recording device. 

In fact you could still record to something like Ableton Live on a laptop (which you wouldn't have to touch at all during performance as all it would be doing is recording audio and midi at this stage) by routing the audio from the 4 deck Pacemaker outputs from two Blackberry Playbooks into Ableton Live as a real time 4 track recording session (although there'd be no way to sync the playbooks to a midi clock). Then using a 4 channel midi mixer in full midi mode you'd be able to record all the control manipulations and changes to the Ableton 4 track recording session that you made on the mixer for easier editing and review later. 

There's also the option to just use it with a 4 channel mixer in analog mode without a laptop and record to some other external recording device as regular none multi-track audio too. In essence two Blackberry Playbooks would become the portable touch based 4 deck setup for my mixer in kind of going all star trek control panel like. As ever there's always the option to use the Pacemaker app on the playbook as a standalone mixing device anywhere (provided you've got enough power in it and considering it can go for 8 to 9 hours with constant full screen 1080 video playback, I'm inclined to think it will last much longer when using the Pacemaker app).

Of course if your passion for DJing and mixing music runs out you've still got a fully featured tablet device that 

can be used for web browsing, email, instant messaging, video messaging/conferencing, games, etc. This also means 

there'd be less waste whilst giving the means to fully digitally mix within a two turntable metaphor. This also means no more teenagers abandoning big heavy turntables and stacks of records in the attic or cellar when they decide they get bored of DJing. The equipment could quite easily be re-purposed for something else and the amount of physical materials required for playbooks would also be far less and much more easily recyclable.

you never realised?...the playbook has become like a western variant of the tablet version of the OLPC but with a recognised brand name on it, it even has decent battery life for running the premium grade hardware. Have you not noticed that you can also buy decent quality 7 to 10 inch android touch tablets (with not so great battery life) for £100 too by lesser known brands with a far larger catalog of applications by sheer virtue of being Android based? Even the premium hardware of Nexus tablets (with decent battery life) gets put out at near the cost of actually making it (although the NEXUS 7 tablet doesn't have separate independent stereo audio outputs like the Blackberry playbook or specialized purpose designed DJ software, maybe buy a PIONEER NEXUS CDJ turntable instead?, Oh wait a minute, that defeats the object of portability and potential ability of mix anytime and anywhere mixing without need for a computer or laptop).

Also regarding sound quality, I can't speak for the Blackberry playbook just yet as I've not tested one (yet), but if the sound quality on the original Pacemaker device was anything to go by (which was excellent from what I recall of actually using one), then the sound reproduction on the Playbook has either got to be on a parallel or better (if it isn't then there's something definitely wrong). This also means it should be equivalent to anything a full sized PIONEER NEXUS CDJ2000 turntable was capable of in terms of audio reproduction when hooked up to an amp/sound system.