Spotify is an audio streaming provider, launched in 2008. From Wikipedia:

Spotify offers digital copyright restricted recorded music and podcasts, including more than 60 million songs, from record labels and media companies. As a freemium service, basic features are free with advertisements and limited control, while additional features, such as offline listening and commercial-free listening, are offered via paid subscriptions. Users can search for music based on artist, album, or genre, and can create, edit, and share playlists.

Spotify is available in most of Europe and the Americas, Oceania, and parts of Africa and Asia; on most modern devices including Windows, macOS, and Linux computers; iOS and Android smartphones and tablets; and AI enabled smart speakers such as Amazon Echo and Google Home. As of October 2020, the platform has 320 million monthly active users, including 144 million paying subscribers.

As of January 1, 2021, the song that has been streamed the greatest number of time on Spotify is "Shape of You" by Ed Sheeran, with 2.695 billion streams. Wikipedia maintains a list of the most-streamed songs.

When will a song have been streamed 4 billion times on Spotify?

This question resolves when Spotify data shows that this has happened. For example, the current web interface shows the number of streams of each artist's popular songs (e.g. Ed Sheeran), and of any track by hovering over its popularity bar. Credible media reports are also sufficient for resolution.

If it is known that this has never happened, then the question resolves as greater than the upper bound. This could happen if Spotify does not continue to exist.

If it is unknown whether this has ever happened, for instance because Spotify does not make this data available, then the question resolves ambiguously.