The plug metaphor doesn't work, because these loads are quite large. 50A circuits can't have 2 outlets. EVSE's can't share circuits. And a 50A appliance needs a 50A breaker to protect it from internal shorts -- if it had a 90A internal short, the 100A breaker wouldn't protect it!
The circuit breaker must match the load(s).
As discussed, each EVSE must have a circuit breaker dedicated to it, and the breaker must be the appropriate size for the EVSE according to its specs and docs. Chevy Volts either use a Level 1 13A/120V charger... or a Level 2 32A/240V charger. Neither of those wants a 50A breaker. Hot-dogging this stuff is not appropriate.
You're always allowed to use a larger wire than is required, so a #4 wire on a 20A circuit is absolutely fine, provided the breaker is 20A and the socket is 20A.
The breaker must protect the wire. (and the loads).
The largest breaker you can use is decided by the wire. We have tables for that, namely 310.15(B)(16). That is based on wire size, wire type (indicating temperature allowed, 60C vs 75C), and copper vs aluminum.
For instance #4 aluminum THWN-2 in conduit is allowed 65A, which you can round up to a 70A breaker.
However, still, the breaker is also limited by the load; e.g. a Level 2 Chevy Volt charger needs a 40A breaker.
The reason is the 125% derate required for EVSEs and other continuous loads. You must provision 125% of the actual current draw. That is why the Volt Level 2 charger is 32A. 125% of 32A is 40A, a common breaker size.
Subpanel is how you solve this.
You can run the wire right up to its 310.15(b)(16) limit if you have it feed a subpanel. Then, in the subpanel, you install breakers appropriate for each EVSE.
We recommend lots and lots of spaces in the subpanel, because spaces are cheap, and running out of spaces is a stupid, expensive problem. So you take our advice and get a 16-space panel with 125A busing. Take that 70A example. That's more than 70A so that's fine.
Then, you install breakers. You install a 40A/240V breaker for your existing Chevy Volt Level 2 EVSE, and a 20A/120V breaker for your level 1 EVSE and general tools. You get the Tesla, and it wants a 50A/240V breaker, so you fit that also.
Now, the 70A breaker in the main panel protects the #4Al wire in the walls, and also protects the 125A busing in the subpanel. Each EVSE breaker protects its individual EVSE.