A method of managing or processing MIDI note data in which the note messages for the notes in a chord are transmitted or received on a separate channel for each note. One shortcoming in the way that MIDI designates notes is that, being keyboard-oriented, it assumes that each note in a chord is unique. For instance, on a conventional keyboard, there is no way of playing two separate middle-C notes at the same time. However, on a guitar, to name an example, it is quite possible to play a chord such that two (or more) strings are playing middle-C at the same time. The guitarist performing on a guitar synthesizer expects that the synth will track and reproduce both. This is a problem in MIDI since there is no way to distinguish between two notes that both have the same note number.
Mode 4 provides a way around this, by dividing up the notes in a chord between separate MIDI channels. This is ideal for a guitar synth; assigning each string to its own MIDI channel ensures that there is no confusion when two strings are sounding the same note. Plus, it allows control on a per-note basis of a number of parameters that are associated with the channel rather than an individual note. For instance, a pitch wheel message normally effects all notes in the currently played chord, but a guitarist has several techniques for bending just one note in a chord. By dividing the notes among channels, the pitch wheel message can be sent for the string that is being bent, and only that note is effected.
Mode 4 is not commonly implemented, but a number of Oberheim's later synth models support it. (They refer to it as "guitar mode" in the documentation.) Soft synths can also often support it by running a separate instance for each MIDI channel. Most recent guitar synths generate MIDI data using Mode 4 by default.
The term "Mode 4" came from the original version of the MIDI specification published by Sequential Circuits, in which they described four channel modes built from combinations of omni mode and polyphonic modes.