From the GitHub release page:

This is it (again)! We added a number of last minute fixes and improvements due to PyCon 2019 sprints. 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 Release Candidate 1

• Auto-complete now omits private variables until an _ has been typed. Thanks to @katlings and @deshipu
• Auto-complete inserts spaces when no matches are found. Thanks to @scottbelden
• Reduce M0 Express build size by removing double float math. Thanks to @dhalbert
• Status LED is blue on MPY version mismatch. Thanks to @terriko
• ReloadExceptions no longer show a traceback. Thanks to @tannewt
• on_next_reset now works with safe mode again. Thanks to @tannewt and @hexthat
• Fix crash on nRF boards with internal flash. Thanks to @tannewt and @jerryneedell
• Check int packing into bytes. Thanks to @godlygeek
• Fix displayio.Group scaling. Thanks to @tannewt and @caternuson
• Network fixes thanks to @nickzoic and @gvcp
• Doc updates thanks to @saxenanurag and @mchua

New and Updated Translations

• More Italian translations. Thanks to @fezthedev
• Spanish translation improvements. Thanks to @rafa-gould, @erikriver, and @sabas1080

New Boards

• Electronic Cats Escornabot Makech. Thanks @sabas1080
• Adafruit PyGamer. Thanks to @ladyada
• Robotics Masters Robo HAT MM1. Thanks to @wallarug

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, @erikriver, @fezthedev, @furlaneto, @gcvasconcelos, @glennrub, @godlygeek, @gpshead, @gvcp, @hathach, @hexthat, @iraytrace, @ITACAInnovation, @Jahor, @jasonp, @jepler, @Jay Doscher, @jerryneedell, @jimblom, @JohnPark, @JoshuaLowe1002, @jreese, @juanita, @KacperLA, @katlings, @kattni, @kdb424, @kevinjwalters, @klardotsh, @Klowner, @ladyada, @larsks, @madbodger, @maholli, @Makdaam, @MakerMelissa, @MattyT, @mchobby, @mchua, @mgithubreq, @microbuilder, @MikeBarela, @Mr. Certainly, @mrmcwethy, @nickzoic, @notro, @ntavish, @oldcrow, @penguindustin, @PhredFL, @platipo, @ptorrone, @rafa-gould, @Rathmore, @rdagger, @Retoc, @rhooper, @sabas1080, @sajattack, @sajattack, @saspa, @saxenanurag, @schmarty, @scottbelden, @Senuros, @ShawnHymel, @siddacious, @sommersoft, @SouthernDragon, @TangoJuliett, @tannewt, @Tasm-Devil, @terriko, @TG-Techie, @ThomasRKirkland, @uhrheber, @urish, @vgoodwinv, @xpecex, @yeyeto2788, @wallarug, @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-rc.1.
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.