  An {\it ab initio} calculation of the $^4$He$(e,e^\prime p)^3$H longitudinal
response is presented. The use of the integral transform method with a Lorentz
kernel has allowed to take into account the full four--body final state
interaction (FSI). The semirealistic nucleon-nucleon potential MTI--III and the
Coulomb force are the only ingredients of the calculation. The reliability of
the direct knock--out hypothesis is discussed both in parallel and in non
parallel kinematics. In the former case it is found that lower missing momenta
and higher momentum transfers are preferable to minimize effects beyond the
plane wave impulse approximation (PWIA). Also for non parallel kinematics the
role of antisymmetrization and final state interaction become very important
with increasing missing momentum, raising doubts about the possibility of
extracting momentum distributions and spectroscopic factors. The comparison
with experimental results in parallel kinematics, where the Rosenbluth
separation has been possible, is discussed.
