  Protoclusters are the progenitors of massive galaxy clusters. Understanding
the properties of these structures is important for building a complete picture
of cluster formation and for understanding the impact of environment on galaxy
evolution. Future cosmic microwave background (CMB) surveys may provide insight
into the properties of protoclusters via observations of the thermal Sunyaev
Zel'dovich (SZ) effect and gravitational lensing. Using realistic
hydrodynamical simulations of protoclusters from the Three Hundred Project, we
forecast the ability of CMB Stage 4-like (CMB-S4) experiments to detect and
characterize protoclusters with observations of these two signals. For
protoclusters that are the progenitors of clusters at $z = 0$ with $M_{200c}
\gtrsim 10^{15}\,M_{\odot}$ we find that the S4-Ultra deep survey has a roughly
20\% chance of detecting the main halos in these structures with SNR > 5 at $z
\sim 2$ and a 10\% chance of detecting them at $z \sim 2.5$, where these
probabilities include the impacts of noise, CMB foregrounds, and the different
possible evolutionary histories of the structures. On the other hand, if
protoclusters can be identified using alternative means, such as via galaxy
surveys like LSST and Euclid, CMB-S4 will be able to obtain high
signal-to-noise measurements of their stacked lensing and SZ signals, providing
a way to measure their average mass and gas content. With a sample of 2700
protoclusters at $z = 3$, the CMB-S4 wide survey can measure the stacked SZ
signal with a signal-to-noise of 7.2, and the stacked lensing signal with a
signal-to-noise of 5.7. Future CMB surveys thus offer exciting prospects for
understanding the properties of protoclusters.
