  The spectral function of 4He is calculated with the Lorentz integral
transform method in a large energy and momentum range. The excitation spectrum
of the residual 3N-system is fully taken into account. The obtained spectral
function is used to calculate the quasi elastic longitudinal (e,e') response
R_l of 4He for q=300, 400, and 500 MeV/c. Comparison with the exact R_l shows a
rather sizeable disagreement except in the quasi elastic peak, where the
differences reduce to about 10% at q=500 MeV/c. It is shown as well that very
simple momentum distribution approximations for the spectral function provide
practically the same results for R_l as the exact spectral function.
