This will hurt Google and may destroy Spotify. If they’re the only ones that get the 0/4% deal and the other services like tidal are paying full price that’s a blatant anti-competitive agreement.
For the same reason you buy nicer toilet paper for your home than the stuff you find at a truck stop: it’s not a significant portion of their expenses.
They could be fined and then forced to pay full whack.
But I'm not sure they meet the threshold for anti-competative practice. You'd need to show some sort of paid exclusivity (eg a bribe so only Spotify could get a better rate).
30% for market access is a bigger net evil that anything Spotify might be doing IMO.