At $8 per month for Prime members, Amazon Music Unlimited was already more affordable than Spotify Premium or Apple Music, but Amazon’s sweetening the pot for new members by offering four months for free, if you have an Amazon Echo device tied to your account.
This is the best deal they’ve ever offered on the service, and it’s even available to non-Prime members. If you have an Echo and you’ve never been a Music Unlimited subscriber, there’s no reason not to sign up.