This is a grand new release, with over two years of work behind it. A lot has changed since version 3.x. Thousands of glyphs have been redesigned. Here are some highlights, compared to the previous version:

• Six additional "Display" designs, assigned to an opsz variable-font axis, exposed as a second family "Inter Display" as static fonts.
• Several new OpenType features
• Humanist-style italics
• Many technical changes (UPM changed to 2048, refined metrics, many improved features, etc)

Compatibility with version 3

Even though the UPM (the font's coordinate system) has changed along with metrics, I've done my best to make sure existing uses of Inter version 3 will look as close to identical as possible for small text sizes, in particular run length.

However you should expect to see noticeable differences when switching to Inter version 4.