Identity
Acetyl heptapeptide-4 is a synthetic acetylated peptide — a chain of seven amino acids with an acetyl group on the end — marketed as a "neuropeptide" for the appearance of expression lines. It belongs to the same cosmetic family as the far better-known acetyl hexapeptide-8 (argireline), and the two are constantly confused, so the first job of this entry is to separate them.
They are not the same molecule. Argireline is a hexapeptide (six amino acids); acetyl heptapeptide-4 is a heptapeptide (seven). Different sequence, different INCI name, different evidence. Whatever modest results argireline has in its small human studies do not transfer to this peptide, and its grade is set on its own thin evidence — see the argireline entry for the more-studied cousin.
Mechanism (as proposed)
The marketing premise is "neuromuscular": that the peptide interferes with the signalling that drives repeated facial movement, so expression lines look softer. The problem is the same one that limits argireline — a peptide has to cross the stratum corneum and reach the neuromuscular junction to relax any muscle, and large peptides do this poorly at the low concentrations used in cosmetics.
What a serum can honestly offer is an appearance-level effect: a smoother-looking surface, perhaps a slightly softened look to movement lines, in the same register as other conditioning cosmetic peptides. What it cannot honestly claim is muscle relaxation or a "topical Botox" outcome — those are medical comparisons with no supporting head-to-head evidence, and muscle-freezing and wrinkle treatment sit outside cosmetic scope. Judge it as a gentle appearance peptide, and keep the wrinkle-erasing expectations with interventions that actually reach the muscle.