The capacity of a vector cannot be controlled by the constructors – there is no applicable overload.
The C++ standard doesn’t give any guarantee about the capacity of a default-constructed vector vector<Foo> bar;. However all well-known implementations use 0 as the default capacity. This is something you can rely on, as allocating memory at that point just doesn’t make sense.
So I believe the answer to your question is: just use
vector<Foo> bar;
bar.reserve(4);