  The spectral function of He-4 is extracted from a plane-wave approximation to
the (e,e'p) reaction using a fully relativistic formalism. We take advantage of
both an algebraic ``trick'' and a general relativistic formalism for quasifree
processes developed earlier to arrive at transparent, analytical expressions
for all quasifree (e,e'p) observables. An observable is identified for the
clean and model-independent extraction of the spectral function. Our simple
relativistic plane-wave calculations provide baseline predictions for the
recently measured, but not yet fully analyzed, momentum distribution of He-4 by
the A1-collaboration from Mainz. Yet in spite of its simplicity, our approach
predicts momentum distributions for He-4 that rival some of the best
nonrelativistic calculations to date. Finally, we highlight some of the
challenges and opportunities that remain, both theoretically and
experimentally, in the extraction of quasifree observables.
