Myth check
不 is always fourth tone.
不 changes to second tone before another fourth tone in common speech.
Verdict
mostly_false
Why
不是 is pronounced bú shì Keep evidence from your own listening and speaking practice, not from fear stories.
Citation tone charts list bù, but sandhi is required in phrases.
Listening for bú prevents robotic speech Keep evidence from your own listening and speaking practice, not from fear stories.
Test the better framing with a short practice drill the same day you read this myth.
Better framing
Practice 不 with tone-4 verbs and 是 from day one.
Practice tip
Practice tip: after reading “不 is always fourth tone.”, do one concrete drill from the better framing—speak aloud for five minutes using related words or tones.