Even Stage 4 isn't granted it will end up on the standard, some proposals have died on Stage 4.
Any example? Since stage 3 is roughly "request for implementers", and Stage 4 requires 2 real world implementations and essentially mean "will be part of the standard in the next wave".
The closest thing I can think of is ES6 modules, because the module loader spec wasn't finished, but the syntax still lived on.
Now, stuff getting kicked down from Stage 3, that has happened.
In terms of chances of standardization, that's a step up from "Hey, I have a great idea", buy there are plenty of Stage 0 proposals that go nowhere because they lose steam.
I think the metric they look for is who is using the Babel plugin for it. If people are doing that, then they it has a better shot of progressing.
Stage 4 is the "this is getting standardized next time" stage