An improvement of the transport critical current density, Jc, of MgB2 wires was obtained after the addition of 10 wt% B4C powders, after reaction at 800 °C: Jc values of 1 × 104 A cm−2 at 4.2 K and 9 T were obtained for wires of 1.11 mm diameter in a Fe matrix. The starting mixture of Mg and B was doped with submicrometric B4C, the ratio being Mg:B:B4C = 1:2:0.08, corresponding to 10 wt% B4C. For T>800 °C, a decrease of Jc was found, due to the reaction with the Fe sheath. In order to investigate the origin of the improvement of the transport properties for heat treatments up to 800 °C, x-ray diffraction measurements were performed. A decrease of the lattice constant a from 3.0854 to 3.0797 Å was found, thus suggesting an effect of the substitution of carbon on the properties of the wires. A comparison with the literature data shows that the addition of B4C powders leads to the second highest improvement of Jc reported so far after SiC, thus constituting an alternative for future applications.