[en] Full text: One of the most exciting topics in nuclear physics is to study how the hadron properties are modified by the nuclear environment and how such modifications affect the properties of nuclei. Since nucleons and mesons are made of quarks, anti-quarks and gluons, one expects their internal structure to change when placed inside nuclear matter or atomic nuclei. Polarization transfer in quasi-elastic nucleon knockout is sensitive to the properties of the nucleon in the nuclear medium. In our recently completed experiment E03-104 at Jefferson Lab in Hall A we measured the proton recoil polarization in the "4He(e, e'p ) "3H reaction at a Q"2 of 0.8 (GeV/c)"2 and 1.3 (GeV/c)"2 with unprecedented precision. These data complement earlier data between 0.4 and 2.6 (GeV/c)"2 from both Mainz and Jefferson Lab. The measured ratio of polarization-transfer coefficients differs from a fully relativistic calculation, and is well described by the inclusion of a medium modification of the proton form factors as predicted by the quark-meson coupling or chiral quark-soliton models. However, these polarization-transfer data are also well described by strong charge exchange final-state interactions. The measured induced polarizations agree well with the fully relativistic calculation and indicate that these strong final-state interactions may not be applicable. (author)