I review the basic features of four dimensional Z_2 x Z_2 (shift)
orientifolds with internal magnetic fields, describing two examples with N=1
supersymmetry. As in the corresponding six-dimensional examples, D9-branes
magnetized along four internal directions can mimic D5-branes, even in presence
of multiplets of image branes localized on different fixed tori. Chiral
low-energy spectra can be obtained if the model also contains D5-branes
parallel to the magnetized directions.Comment: 4 pages, LATEX; misprints correcte
