  We discuss the effects of an applied magnetic field on the low energy
excitations in the low temperature phase of Yb4As3. We show also why the
magnetic interaction of the Yb{3+} ions is nearly of an isotropic Heisenberg
spin-1/2 type. A small anisotropy due to an intrachain dipolar interaction
leads to the opening of a gap when a magnetic field is applied. The model
agrees with available experimental data. Simple experiments are suggested in
order to further test the present theory.
