  We study the sensitivity of LiteBIRD and CMB-S4 to the reheating temperature
and the inflaton coupling in three types of plateau-potential models of
inflation, namely mutated hilltop inflation, radion gauge inflation, and
$\alpha$-attractor T models. We first find relations between model parameters
and CMB observables in all models. We then perform Monte Carlo Markov Chain
based forecasts to quantify the information gain on the reheating temperature,
the inflaton coupling, and the scale of inflation that can be achieved with
LiteBIRD and CMB-S4, assuming a fiducial tensor-to-scalar ratio $\bar{r} \sim
0.02$ and neglecting foreground contamination of the B-mode polarization
spectrum. We compare the results of the forecasts to those obtained from a
recently proposed simple analytic method. We find that both LiteBIRD and CMB-S4
can simultaneously constrain the scale of inflation and the reheating
temperature in all three types of models. They can for the first time obtain
both an upper and lower bound on the latter, comprising the first ever
measurement of the big bang temperature. In the mutated hilltop inflation and
radion gauge inflation models this can be translated into a measurement of the
inflaton coupling in parts of the parameter space. Constraining this
microphysical parameter will help to understand how these models of inflation
may be embedded into a more fundamental theory of particle physics.
