  The Rubin Observatory's 10-year Legacy Survey of Space and Time will observe
near to 20 billion galaxies. For each galaxy the properties can be inferred.
Approximately $10^5$ galaxies observed per year will contain Type Ia supernovae
(SNe), allowing SN host-galaxy properties to be calculated on a large scale.
Measuring the properties of SN host-galaxies serves two main purposes. The
first is that there are known correlations between host-galaxy type and
supernova type, which can be used to aid in the classification of SNe.
Secondly, Type Ia SNe exhibit correlations between host-galaxy properties and
the peak luminosities of the SNe, which has implications for their use as
standardisable candles in cosmology. We have used simulations to quantify the
improvement in host-galaxy stellar mass ($M_\ast$) measurements when
supplementing photometry from Rubin with spectroscopy from the 4-metre
Multi-Object Spectroscopic Telescope (4MOST) instrument. We provide results in
the form of expected uncertainties in $M_\ast$ for galaxies with 0.1 < $z$ <
0.9 and 18 < $r_{AB}$ < 25. We show that for galaxies mag 22 and brighter,
combining Rubin and 4MOST data reduces the uncertainty measurements of galaxy
$M_\ast$ by more than a factor of 2 compared with Rubin data alone. This
applies for elliptical and Sc type hosts. We demonstrate that the reduced
uncertainties in $M_\ast$ lead to an improvement of 7\% in the precision of the
"mass step" correction. We expect our improved measurements of host-galaxy
properties to aid in the photometric classification of SNe observed by Rubin.
