Broadly speaking, what you're asking for is not really viable.

The problem with simplistic, fixed-bound allocators like these is that
it only works while communicating effectively with the container
itself.

`vector`'s allocation strategy is entirely *opaque* to the user. In
your example, how do you as the user of `vector` know that the first
`push_back` won't try to allocate more than 4 characters? There's no
requirement than, when using `push_back` on an empty `vector`, it will
allocate a buffer of no more than 4 values. There's no requirement
that calling `reserve(x)` will only allocate `x` items.

Everything you want ultimately requires a tighter coupling between the
container and the allocator than C++ provides. The only way to resolve
this is to make some fundamental changes to the allocator/container
model.