No fees when users choose to pay via Spotify (which had been the case and only option since the beginning, until User Choice Billing was implemented).
If users choose to pay with Google Play Billing, Google keeps 4%.
Even so, what I find hypocritical is that Spotify got this deal and seemingly agreed to keep it under wraps, without advocating for it to be extended to all other music streaming services in the platform.
Because... having a deal with the platform holder that gives it unfair advantage over the competition is exactly what they accuse Apple of doing with iOS.
4% is basically just the payment processing fee (averaged out, since it's slightly different for every transaction). Spotify has to pay that regardless of how you pay.