Promote [Unreleased] to [0.4.1] - 2026-04-18, bump @version in
mix.exs, and correct the stale "~> 0.1" installation hint in
README.md to "~> 0.4".

Previous local @version "0.4.0" was never published to hex.pm
(latest published: 0.3.1). Hex users jump 0.3.1 -> 0.4.1.