The forces between nucleons are one of the most studied problems in
physics. With the three-nucleon system being (apart from a few
remaining discrepancies such as the Ay puzzle of dp elastic
scattering) well described by state-of the-art Faddeev calculations,
and with some progress in Faddeev-Yakubovsky calculations, the
interest in the four-nucleon system has grown appreciably in recent
years. Like the three-nucleon system it is a suitable testing ground
for modern meson-exchange nucleon-nucleon forces introduced into these
calculations, as well as for the effects of three-(and four-)nucleon
forces. Recent successes in ab-initio calculations of levels of light
nuclei assign a special role to the four-nucleon scattering system as
intermediate between the two-and three-nucleon system and heavier
light nuclei. The use of polarized beams and targets allows to reveal
much more information in reactions. Experimental studies of reactions
with polarized protons and deuterons at low energies, which have been
performed at tandem Van de Graaff accelerator laboratories in Cologne
and at TUNL, will be presented. Progress in a better understanding of
the 4N system has been made, but open questions remain.