A tetrapeptide is a peptide of exactly four amino acid residues joined by three peptide bonds. Typical molecular weights fall between roughly 350 and 600 Da depending on side chains, which places tetrapeptides well below the usual 500 Da rule-of-thumb ceiling for passive diffusion only when the residues are small and uncharged.
Why this length keeps appearing
Four residues is long enough to present a defined side-chain arrangement and short enough to be synthesised cheaply at high purity by standard SPPS, with few deletion impurities and an unambiguous mass spectrum. That combination explains its recurrence across otherwise unrelated fields: Epitalon (Ala-Glu-Asp-Gly) among Khavinson bioregulators; Rigin and Tetrapeptide-21 in cosmetic formulation; the matrikine fragments used in skin research; and Tetrapeptide-30 in pigmentation work.
Practical consequences follow from the size. Tetrapeptides are rapidly cleared by aminopeptidases unless N-terminally capped, they are usually supplied as acetate or TFA salts where the counter-ion is a significant fraction of the vial mass, and highly polar sequences can be hygroscopic enough to complicate weighing.
Related terms and material
tripeptide · molecular weight (Da) · purity. Reference material: Epitalon, cosmetic signal peptides. Further reading: peptide sequences explained.