Can't install OCaml 4.09, even though it's in Nixpkgs

Solved: I needed to install ocaml as ocamlPackages_latest.ocaml

I can’t seem to install OCaml 4.09, even though it’s in nixpkgs here. I’m subscribed to both nixos-19.09 and nixos-unstable channels, so I’m not sure why I can’t find it. Any ideas? Thanks!