Phoenix won't respond on IPv4

I’m trying to setup a Vagrant box using generic/freebsd12 as the base image. I cannot get my Phoenix application to listen on IPv4 addresses inside the VM. It will respond to IPv6 as expected, so I know it works.

config :bison_app, BisonWeb.Endpoint,
  http: [
    port: String.to_integer(System.get_env("PORT") || "4000")
  url: [host: System.get_env("HOST") || "app.local"],
  cache_static_manifest: "priv/static/cache_manifest.json",
  secret_key_base: secret_key_base

From inside the VM:

$ curl -4 localhost:4000
curl: (7) Failed to connect to localhost port 4000: Connection refused

$ curl -6 localhost:4000
<HTML response>