  A detailed study of the 4He longitudinal response function R_L(\omega,q) is
performed at different kinematics, with particular emphasis on the role of
three-nucleon forces. The effects shown are the results of an ab initio
calculation where the full four-body continuum dynamics is considered via the
Lorentz integral transform method. The contributions of the various multipoles
to the longitudinal response function are analyzed and integral properties of
the response are discussed in addition. The Argonne V18 nucleon-nucleon
interaction and two different three-nucleon force models (Urbana IX,
Tucson-Melbourne') are used. At lower momentum transfer (q<= 200$ MeV/c)
three-nucleon forces play an important role. One even finds a dependence of R_L
on the three-nucleon force model itself with differences up to 10%. Thus a
Rosenbluth separation of the inclusive electron scattering cross section of 4He
at low momentum transfer would be of high value in view of a discrimination
between different three-nucleon force models.
