  NGC7793, NGC300, M33 and NGC2403 are four nearby undisturbed and bulgeless
low-mass spiral galaxies with similar morphology and stellar mass. They are
ideal laboratories to study disc formation scenarios and stellar mass growth
histories. We construct a simple chemical evolution model by assuming that
discs grow gradually with continuous metal-free gas infall and metal-enriched
gas outflow. By means of the classical $\chi^{2}$ methodology, applied to the
model predictions, the best combination of free parameters capable of
reproducing the corresponding present-day observations is determined, i.e. the
radial dependence of the infall timescale $\tau=0.1r/{R_{\rm d}}+3.4\,{\rm
Gyr}$ ($R_{\rm d}$ is the disc scale-length) and the gas outflow efficiency
$b_{\rm out}=0.2$. The model results are in excellent agreement with the
general predictions of the inside-out growth scenario for the evolution of
spiral galaxies. About 80\% of the stellar mass of NGC\,7793 is assembled
within the last 8\,Gyr and 40\% within the last 4\,Gyr. By comparing the
best-fitting model results of the three other galaxies we obtain similar
results, 72\% (NGC300), 66\% (NGC2403) and 79\% (M33) stellar mass were
assembled within the past $\sim\rm 8\,Gyr$ (i.e. $z\,=\,1$). These four disc
galaxies simultaneously increase their sizes and stellar masses as time goes by
and they grow in size at $\sim\,0.30$ times the rate at which they grow in
mass. The scale-lengths of these four discs are now 20\% -- 25\% larger than at
$z\,=\,1$. Our best-fitting model predicted the stellar mass-metallicity
relation and the metallicity gradients, constrained by the observed
metallicities from HII-regions emission line analysis, agree well with the
observations measured from individual massive red and blue supergiant stars and
population synthesis of SDSS galaxies.
