  We present an ultrafast fiber laser system delivering 4.6 W average power at
258 nm based on two-stage fourth-harmonic generation in beta barium borate
(BBO). The beam quality is close to diffraction limited with an M2- value of
1.3 x 1.6. The pulse duration is 150 fs, which potentially is compressible down
to 40 fs. A plain BBO and a Sapphire-BBO compound are compared with respect to
the achievable beam quality in the conversion process. This laser is applicable
in scientific and industrial fields. Further scaling to higher average power is
discussed
