  A search for periodic gravitational waves, from sources such as isolated
rapidly-spinning neutron stars, was carried out using 510 hours of data from
the fourth LIGO science run (S4). The search was for quasi-monochromatic waves
in the frequency range from 50 Hz to 1500 Hz, with a linear frequency drift
f-dot (measured at the solar system barycenter) in the range -f/tau < f-dot <
0.1 f/tau, where the minimum spin-down age tau was 1000 years for signals below
300 Hz and 10000 years above 300 Hz. The main computational work of the search
was distributed over approximately 100000 computers volunteered by the general
public. This large computing power allowed the use of a relatively long
coherent integration time of 30 hours, despite the large parameter space
searched. No statistically significant signals were found. The sensitivity of
the search is estimated, along with the fraction of parameter space that was
vetoed because of contamination by instrumental artifacts. In the 100 Hz to 200
Hz band, more than 90% of sources with dimensionless gravitational wave strain
amplitude greater than 1e-23 would have been detected.
