First things first, its appearance. Even I'll admit that the original Mopho keyboard's bright yellow color scheme wasn't to everyone's taste. The slick black design and wooden end panels of the x4 should cause fewer frowns. The knob and button array of the x4 is otherwise identical to the original Mopho, which is something of a surprise and a bit of a missed opportunity. Envelope controls for filter, amplifier or auxiliary routings still have to be selected via a switch before twiddling a shared set of dials to control attack, decay, sustain and release. But since there's ample space for three lines of envelope dials on the x4—the keyboard length is extended to 44 notes (featuring velocity and aftertouch), so there's a fair bit of panel space left on the right side of the fascia—it would have been great to see DSI assign one knob to each function.
Sonically, the x4's architecture remains the same as on the Mopho keyboard. This means that each note has the capacity to trigger two analog oscillators and two sub oscillators before the signal passes into a low-pass filter switchable between two- or four-pole designs. Then come three envelope generators alongside four LFOs. The onboard engine is capable of providing modulation from 20 sources to more than double the number of destination targets. Obviously, if you take advantage of the x4's maximum polyphony, you get four of all of the above. The sound engine then collectively feeds into the onboard sequencer or arpeggiator. Surprisingly, and somewhat disappointingly, DSI dropped the Mopho keyboard's audio input from the x4. However, the poly chain port remains in place, meaning that you can expand the number of voices from the x4 to any other DSI synthesizer directly. This means you could chain up a Tetra for four more voices or expand the voice count even further with a Prophet '08 or 12.
As with all of DSI's current range of synths, the sound is sumptuous, which makes the option to play chords hugely welcome. It opens the instrument up to a wider range of sounds than the original Mopho keyboard could hope to provide. While this keyboard could be front-and-center on practically any production, I also found it wonderful for creating soft, filtered, barely-there pads which can provide glue for your mixes. In that sense, this is a sound engine with multiple personalities, which hugely increases its potential appeal.
I wonder, however, how the Mopho x4 stacks up against some of the other instruments in DSI's arsenal. The Prophet '08 is only a few hundred dollars more expensive, with the benefits of yet more polyphony and expanded sonic options to boot (albeit at the expense of a keyboard). Equally, the Tetra module offers four voices and multi-timbrality for around two-thirds of Mopho x4's price (this time with the sacrifice of much of the x4's instant tactility). So definitely do your research before planning which DSI synthesizer suits you best. The Mopho x4, though, should make the decision that much harder. It's not a bad problem to have.
Ease of use: 3.5/5