In terms of metadata privacy, NIP-17 is better than NIP-4. NIP-17 provides better protection for the sender's ID, but it does not sufficiently protect the recipient's ID. The visibility of someone continuously receiving messages is also a metadata privacy issue. For a DM (a feature subsidiary to microblogging), this might be an acceptable tradeoff, as users can receive and decrypt messages on multiple clients. However, for an application using Nostr for a chat app, privacy should be a priority.
Discussion
DM (a subsidiary feature of microblogging) and chat apps (primarily focused on chatting) are not exactly the same, and they face different tradeoffs. A chat app needs to consider end-to-end encryption with both forward and backward secrecy, as well as the ability to conceal the IDs of both the sender and the recipient.