The North American release Avengers 4 may have been moved up by at least one week. Originally slated to hit North American theaters May 3, 2019, a quarterly financial report released by IMAX lists the highly anticipated follow-up to Infinity War for April 2019.
Although Avengers 4 is already releasing in the United Kingdom in April, the IMAX schedule lists movies by their domestic release dates, suggesting the blockbuster has, indeed, been moved up stateside. Furthermore, as ComicBook points out, IMAX makes notes for "international markets" for any exceptions made on the schedule; there are no mentions included for Avengers 4.
Marvel has not officially announced any change in dates, but assuming the IMAX schedule is correctly updated with the new release window, it's likely that the North American release has been moved up to coincide with the film's UK release on April 26, 2019.
Marvel movies typically have a staggered release, with many of them dropping a week early in the UK; although, there have been exceptions as the studio tries to select dates that will best position its movies for success at the box office. This wouldn't be the first time Marvel does something like this, either. In an attempt to avoid potential spoilers, the studio also moved the domestic release of Avengers: Infinity War up one week to match the UK. Again, assuming IMAX's date is correct, it's possible Marvel is doing the same thing with Avengers 4. Or perhaps, with Infinity War racking up over $2 billion at the box office, Marvel is just fond of April releases.
Hopefully, Marvel will make an official announcement. Disney's earnings report is scheduled for August 7 which could be when any shift in dates is formally announced. In the meantime, Avengers 4 will release in North American theaters in April 2019, or possibly May still.