The relationship between NIP-4 and NIP-17 is similar to: iPhone ↔ iPhone: It uses iMessage first (Apple’s service). Blue bubbles. iPhone ↔ Android: It uses SMS/MMS (the carrier’s traditional texting standard). This isn’t an “Android-only protocol,” it’s the old common language that all phones can speak. Green bubbles. If at least one person in the chat is using a client that supports both NIP-4 and NIP-17, interoperability is no longer an issue.