If you installed opam
on Fedora 28, you might be surprised the fact that you can't do opam switch 4.0x.x
.
On Fedora 28, sudo dnf install opam
installs opam 2.0 beta (as of 2018-06-02).
Official Blog says that there are some changes to be aware of, including:
opam switch create is now needed to create new switches
Therefore, what you want to do should be like this:
opam switch list-available
opam switch create 4.x.x