Narrow Results

The four-nucleon bound state and scattering below three-body breakup threshold are described. The description is based on a realistic coupled-channel potential which allows the excitation of a single nucleon to a Δ isobar. In the four-nucleon system the Δ isobar mediates effective two-, three- and four-nucleon forces. The effect of the four-nucleon force is found to be much smaller than the effect of the three-nucleon force.