system doesn't comply with ocaml-version >= "4.03"

[idkjs]

macOS Sierra: ocsigen-toolkit successfully installed.
Can not install ocsigen start.

$ opam install ocsigen-start
[ERROR] ocsigen-start is not available because your system doesn't comply with
        ocaml-version >= "4.03".

New to ocaml. Any ideas on what I need to do to comply? Current opam version is: opam 1.2.2_3 already installed. And this is ocaml, installed via homebrew:

brew info ocaml
ocaml: stable 4.05.0 (bottled), HEAD
General purpose programming language in the ML family
https://ocaml.org/
/usr/local/Cellar/ocaml/4.05.0 (1,989 files, 206.7MB) *
  Built from source on 2017-09-14 at 03:36:11
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/ocaml.rb
==> Requirements
Optional: x11 ✘
==> Options
--with-flambda
	Install with flambda support
--with-x11
	Install with the Graphics module
--HEAD
	Install HEAD version