Text this: The Immunosuppressive Properties of Cyclo-[<i>D</i>-Pro-Pro-<i>β</i><sup>3</sup>-HoPhe-Phe-] Tetrapeptide Selected from Stereochemical Variants of Cyclo-[Pro-Pro-<i>β</i><sup>3</sup>-HoPhe-Phe-] Peptide