  We present upgraded infrastructure for Searches after Gravitational Waves
Using ARizona Observatories (SAGUARO) during LIGO, Virgo, and KAGRA's fourth
gravitational-wave (GW) observing run (O4). These upgrades implement many of
the lessons we learned after a comprehensive analysis of potential
electromagnetic counterparts to the GWs discovered during the previous
observing run. We have developed a new web-based target and observation manager
(TOM) that allows us to coordinate sky surveys, vet potential counterparts, and
trigger follow-up observations from one centralized portal. The TOM includes
software that aggregates all publicly available information on the light curves
and possible host galaxies of targets, allowing us to rule out potential
contaminants like active galactic nuclei, variable stars, solar-system objects,
and preexisting supernovae, as well as to assess the viability of any plausible
counterparts. We have also upgraded our image-subtraction pipeline by
assembling deeper reference images and training a new neural network-based
real-bogus classifier. These infrastructure upgrades will aid coordination by
enabling the prompt reporting of observations, discoveries, and analysis to the
GW follow-up community, and put SAGUARO in an advantageous position to discover
kilonovae in the remainder of O4 and beyond. Many elements of our open-source
software stack have broad utility beyond multimessenger astronomy, and will be
particularly relevant in the "big data" era of transient discoveries by the
Vera C. Rubin Observatory.
