The D4 is a four-track digital recorder with built-in effects, a simple rhythm generator, and some basic audio editing tools. It's a touch smaller than a sheet of A5 paper, no deeper than a cheese sandwich, and about as heavy as a copy of Sound On Sound. As you can imagine, it's pretty compact, although too large to be a pocket device, especially when its external 9V power supply is taken into consideration. In fact the D4 is more of a 'recording notepad', capable of recording basic compositions two tracks at a time, and requiring the minimum of outboard. For example, a guitarist can start by plugging their lead directly into the back, use the in-built tuner to get set up, select a suitable amplifier and speaker simulation, lay on some reverb, delay or distortion, and play along to one of the internal rhythms at a tempo and time signature of choice. The D4's editing tools make it possible to hone a performance somewhat, alternative takes may be stored on the 28 virtual tracks, and there are additional send effects available for use on the mix and across the stereo buss during mixdown. Complete mixes can be saved as stereo MP2 files and imported into a USB-connected PC or Mac, or simply output to a stereo recorder of choice.