Cramming three or four independent voices onto a SINGLE stave (not the SATB two-and-two split) is almost never correct engraving

A single stave holding 3-4 concurrently-notated voices — as opposed to the SATB convention of splitting them 2-and-2 across two staves — produces stacked opposing-stem chords with no reliable way to tell which notehead belongs to which voice, since VexFlow/MusicXML stem-direction and beam conventions only cleanly disambiguate TWO roles (up/down) per stave. Gould states short-score (2-voices-per-stave) writing as the practical ceiling for a single stave; anything beyond that belongs on separate staves.

Gould, Behind Bars, ch. on multiple voices — short score (max 2 voices per stave) is the stated convention; Read, Music Notation, and Ross, The Art of Music Engraving, concur that 3+ real voices sharing one stave is reserved for exceptional, brief keyboard-reduction contexts, never for continuously independent lines.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.