In order to study the origin of the strong Lyα emission of high-redshift starburst galaxies we observed and modeled the rest-frame UV spectrum and the Lyα emission of the z = 3.304 galaxy FDF-4691 (rest-frame EWLyα = 103 Å). The observations show that FDF-4691 is a young starburst galaxy with a (for this redshift) typical metallicity. The broad, double-peaked profile of the Lyα emission line can be explained assuming a highly turbulent emission region in the inner part of the starburst galaxy, and a surrounding extended shell of low-density neutral gas with a normal dust/gas ratio and with Galactic dust properties. The detection of the Lyα emission line is explained by the intrinsic broad Lyα emission and a low HI column density of the neutral shell. A low dust/gas ratio in the neutral shell is not needed to explain the strong Lyα line.Based on observations obtained at the ESO VLT at Cerro Paranal, Chile.