From the GitHub release page:

This is it! We think this is stable but want you to test it out to be sure. If we don’t find anything major, then this will be the 4.0.0 release. If there are any fixes to make we’ll follow up with another release candidate.

If you find a bug please check the current known issues and file an issue if something isn’t already known.

circuitpython.org

Downloads are now available from circuitpython.org! This site makes it much easier to select the correct file and language for your board. The downloads page is here.

4.0.0

4.0.0 is the latest major revision of CircuitPython. It features:

• extensive new support for pixel-based displays (displayio)
• a port to the Nordic nRF52840 microcontroller, including support as a BLE beacon or peripheral (bleio)
• USB MIDI support
• messages translated into multiple languages

Installation

To install follow the instructions in our new Welcome to CircuitPython! guide. To install the latest libraries, see this page in that guide.

Try the latest version of the Mu editor for creating and editing your CircuitPython programs and for easy access to the CircuitPython serial connection (the REPL).

New Features, Updates, and Fixes since Beta 7

• nrf: Added nvm support. Thanks to @nickzoic and @dhalbert
• Add USER_C_MODULE support. Thanks to @dmazzella.
• Added gamepadshift for buttons connected to a shift register. Thanks to @deshipu and @tannewt
• Refreshed the README and fixed PDF build. Thanks to @tannewt
• Clarified I2SOut error messages. Thanks to @C47D and @tannewt
• Fixed backlight flickering. Thanks to @dhalbert
• Update pybadge pinout for production revision. Thanks to @ladyada
• Numerous crash fixes. Thanks to @tannewt

New and Updated Translations

• Tweaks to Polish translation. Thanks to @deshipu
• Additional Chinese (Pinyin) translations. Thanks to @hexthat
• Tweaks to French translation. Thanks to @arofarn

New Boards

• Electronut Labs Papyr. Thanks @ntavish and @ITACAInnovation

Breaking Changes from 3.x!

• ure, ujson, and uerrno renamed to re, json, and errno to reflect that they are subsets of the CPython library modules. Thanks to @dhalbert.
• busio.UART breaking changes to match PySerial. Thanks to @dhalbert.
  • busio.UART constructor timeout parameter is now in seconds instead of milliseconds.
  • UART.readinto(buf) now does not allow an optional length parameter UART.readinto(buf, len).
  • UART.write(…) no longer allows strings to be written. The bytes written must be bytes or bytearray or an array.array.
• framebuf has been removed. displayio is its replacement, and will be enhanced after 4.0 to include framebuf‘s functionality. Thanks to @tannewt.
• Improve struct compatibility with CPython by enforcing stricter types. This may break your code! Thanks to @dhalbert.

Thanks

Thank you to all who used, tested, contributed, helped out, and participated on GitHub and/or Discord:
@adafruit, @alwayslinux2, @anecdata, @arofarn, @arturo182, @ATMakersBill, @barbudor, @brennen, @brentru, @bwshockley, @C47D, @CarlFK, @carlosperate, @caternuson, @CedarGroveStudios, @chalei, @charlesbunaford, @cpforbes, @CraigF, @dastels, @deshipu, @dhalbert, @dsiee, @dmazzella, @edspark, @eighthree, @emteeoh, @furlaneto, @gcvasconcelos, @glennrub, @gpshead, @gvcp, @hathach, @hexthat, @iraytrace, @ITACAInnovation, @Jahor, @jasonp, @jepler, @Jay Doscher, @jerryneedell, @jimblom, @JohnPark, @JoshuaLowe1002, @jreese, @juanita, @KacperLA, @kattni, @kdb424, @kevinjwalters, @klardotsh, @Klowner, @ladyada, @larsks, @madbodger, @maholli, @Makdaam, @MakerMelissa, @MattyT, @mchobby, @mgithubreq, @microbuilder, @MikeBarela, @Mr. Certainly, @mrmcwethy, @nickzoic, @notro, @ntavish, @oldcrow, @penguindustin, @PhredFL, @platipo, @ptorrone, @Rathmore, @rdagger, @Retoc, @rhooper, @sabas1080, @sajattack, @sajattack, @saspa, @schmarty, @Senuros, @ShawnHymel, @siddacious, @sommersoft, @SouthernDragon, @TangoJuliett, @tannewt, @Tasm-Devil, @TG-Techie, @ThomasRKirkland, @uhrheber, @urish, @vgoodwinv, @xpecex, @yeyeto2788, @wawachief, and surely more we have missed. Join us on the Discord chat to collaborate.

Documentation

Documentation is available in readthedocs.io.

Here are all the changes since 4.0.0-beta.7.
Here are all the changes since 3.1.2.

This release is based on MicroPython 1.9.4 @25ae98f. Support upstream MicroPython by purchasing a PyBoard (from Adafruit here).

Troubleshooting

Check out this guide for info on common problems with CircuitPython. If you are still having, then post to the Adafruit Support Forums and join Discord.

Note: 4.0.0-rc.0 was tagged but not really released. We wanted to get pybadge changes into the RC that make it work with the production board.