The Jupiter-4's basic architecture consisted of four identical voice cards, each with a VCO (and sub-oscillator), resonant low pass VCF (Roland BA662 in earlier revisions or IR3109 in later models IC, which could self-oscillate), and variable-gain amplifier (VCA). Modulation included an Attack-Decay-Sustain-Release (ADSR) envelope for the filter, and another for the voltage-control amplifier and a final level output with an overload LED, as well as a separate unmemorized master volume control. The filter ADSR could be inverted, allowing for "upside down" modulation. The final VCA level setting could be memorized in user presets, and was prior to the overall master stereo output volume. The LFO, routable to oscillator pitch, pulse width, filter cutoff and amplifier, was notable for being able to reach audio frequencies, allowing for crude FM and AM synthesis. Those settings were memorized in the user presets but a fully adjustable depth remained independently configurable through the pitch wheel to combinations of VCO, VCF and VCA, as well as a bend range. The VCOs can garner unstable tuning if aging or low-quality electrolytic capacitors are used (two for each voice card). Synthesizer repair shops can replace these tuning capacitors with stable polystyrene capacitors for an instant perfect tune. Individual oscillator card VCO tuning is accessed by four capped holes in the middle of the back of the case, but the unit should be allowed to warm up before adjusting for at least 20 to 40 minutes prior to adjustment.
The Jupiter 4's two most distinctive features were provided by virtue of its "compuphonic" digital control of the four voice cards, made possible by two Intel 8048 chips:
An arpeggiator, with a choice of up, down, up/down, or random mode. Switches on the far left select between the internal rate, adjustable, or an external source, such as the clock out of its contemporary, the Roland CR-78—Roland's first programmable analog drum machine. A hold button on the lower front panel allows users to latch or set a constantly running arpeggiator pattern, useful when playing leads on another machine over a JP-4 pattern. Otherwise the arpeggiation only responds when keys, individual or chords, are pressed. This allows for a more fluid and less rigid timing in live situations when playing with others.
Left-hand modulation from the keyboard is unusual. The polyphonic portamento or glide feature can be used very effectively with in conjunction with the arpeggiator and any preset. An octave down switch by the modulation wheel is also available independent of memorized settings. Unlike Moog or Sequential, the Roland modulation wheel goes left to right, and is spring-loaded. A short spike on top of the spring-loaded modulation wheel allows for an unusual rapid fanning, but risks damage to this out of production part. Knobs allow depth of pitch bend or LFO modulation, in addition to the amounts assigned and memorized. Toggle switches select bend or LFO to the wheel and onto any combination of VCO/VCF/VCA in an unusual selection pattern, all controlled by a single wheel motion.
Four voice assignment modes, which, as well as simple one-VCO-per-voice polyphony, included the ability to affect four-VCO unison when one key was pressed, two VCOs per voice when two keys were pressed, and one VCO per voice when three or four keys were pressed.
The final signal path also included a simple high-pass filter and a lush stereo chorus effect based on two, now rare, MN3004 ICs. The chorus circuit board is located underneath the modulation wheel, and has one control: a front button for enabling/disabling the effect. It is lush and wide, supplying a pseudo stereo effect when both outputs are used. This filter, as well as other parts of the machine, also use the BA662 VCA chips, which are also rare. Despite evidence against its alleged sonic magic, the BA662 remains sought by many x0xb0x builders, a superior clone DIY kit of the Roland TB-303. Numerous such chips are used in the early JP-4 circuits and the Roland TB-303.
The Jupiter 4 had ten preset sounds and also featured eight memory locations for user-created patches. Saving to those locations requires two widely separated write record buttons to be held, to protect against accidental writing. A battery located deep between the chorus and power supply preserves the contents of the memory ICs.
A quirk that catches many first-time users is that the lower right preset and user memory buttons disengage most programmable controls to the right of the arpeggiator section when a user or factory preset is used. The controls to the right of the arpeggiator are only live when the yellow manual button is selected. The controls are analog switches and potentiometers, but do not adjust the synth cards directly. They are supplied with hi/low logic voltages or 0–5 volt voltages. Multiplexers and analog to digital converters read the settings, then send them in digital form to the CPU. The CPU then converts these parameters to analog control voltages, and sends them to the analog voice cards and envelope gates. It is a remarkably stable hybrid pioneer with a great analog sound.
Despite not being incredibly popular, it did find its way into the hands of some musicians, most of which were associated with the new wave and synthpop music scenes.