Paramount has already announced a release date for the fourth Sonic the Hedgehog movie, despite the fact that the third movie was only made available to stream yesterday (and that it's still available to watch in some theaters).
Per Variety, Sonic the Hedgehog 4 will be launching on March 19th, 2027. Obviously, that date is more than two years away, but hey, I suppose Sonic movie fans need something to look forward to.
Sonic the Hedgehog 4 won't necessarily be the movie's final title, but if it follows the naming conventions of its two predecessors, then it probably won't bear any kind of secondary name. Don't expect Sonic 4: The Rise of Robotnik or anything anytime soon.

A report back in December (also courtesy of Variety) already revealed that a fourth Sonic movie was confirmed before the proverbial ink on the third one was dry, but now we know exactly when we can expect it.
Of course, we still don't know what kind of story details we can expect from the movie, although the post-credits scenes included in the third installment are probably strong clues as to where it's headed.
The third movie's post-credits scenes (look away now if you don't want spoilers!) confirm that Shadow survived the events of the film, and they also tease the appearance of both Metal Sonic and Amy Rose in the next movie.
As such, we can probably expect Sonic 4 to focus on both characters, and Keanu Reeves' Shadow the Hedgehog will probably be returning as well. Naturally, all of this is speculation until the movie is officially revealed.
The existence of a fourth Sonic the Hedgehog movie makes a lot of sense given how well the previous three have done in financial terms.
According to Paramount, between them, the three Sonic films have crossed $1 billion at the box office, and since that report was released earlier this month, that number's likely to be higher by now as well.
In any case, Sonic the Hedgehog 4 releases on March 19th, 2027. Don't expect to hear anything more about the movie for a while given how far off that date is.