How many versions of clang/llvm are currently required to maintain a system

I've finally been able to bootstrap CURRENT FreeBSD on
some new hardware I picked up -- that is, successfully
get world/kernel built/installed on it.
I see there's a newer version of clang in base (4),
which I had hoped that ports would pick up on, and use.
But I suppose it's a bit early for ports to start
requiring 4. That said; given the time it takes to
build install fresh version(s) of these. Is there any
way to limit the version /chosen/ by the ports
tree/framework? It's painful firing up a build of a
meta-port, and watch more than one version of clang/llvm
be built for the BUILD_DEPENDS, only to have it cleaned
and never installed, only to be built again, and again...
You get the picture. :-)
Anyway, as I indicated; I'm hoping to limit the ports
framework to building/using only /one/ version;
say >=3.9?

I'd simply look through the system I have, but it's
in a build session, that looks like it won't be
finished for more than an hour.

Thank you for all your time, and consideration.

--Chris