  Gravitational waves from binary neutron star mergers can be used as alerts to
enable prompt follow-up observations. In particular, capturing prompt
electromagnetic and astroparticle emissions from the moment of a binary merger
presents unique constraints on the time scale and sky localization for online
gravitational wave detection. Here we present the expected performance of the
SPIIR online detection pipeline that is designed for this purpose in the
upcoming international LIGO-Virgo's 4th Science Run (O4). Using simulated
Gaussian data for the two LIGO observatories with expected O4 sensitivity, we
demonstrate that there is a non-negligible opportunity to deliver pre-merger
warnings at least 10 s before the final plunge. These alerts are expected to be
issued at a nominal rate of one binary neutron star coalescence per year and
localized within a median searched area of 300 $deg^2$. We envision such a
detection to be extremely useful for follow-up observatories with a large field
of view such as the Murchison Widefield Array radio facility at Western
Australia.
