From the GitHub release page:

This is a beta build of 4.0.0. There are still issues to fix before stable and the new APIs may evolve a bit more. Please use the latest 3.x stable release when first starting with CircuitPython. It is stable.

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

4.0.0

4.0.0 is the latest major revision of CircuitPython and features support for the nRF52840 BLE chipset and translated messages.

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).

Most builds are available as both UF2 and bin files here along with test builds.

New Features, Updates, and Fixes since Beta 1

• nrf: Add touchio support to nRF52840 using a 1mOhm drain resistor. Thanks to @nickzoic
• nrf: Improve USB and BLE coexistence. Thanks to @hathach
• displayio changes. Thanks to @tannewt
  • Adds serial output to displays when they are available.
  • Replaces Sprite with TileGrid which is more generic.
  • Display now manages the backlight brightness. To change it manually do display.auto_brightness = False and then set display.brightness with a value 0 to 1.0.
  • Add support for display rotation.
• Enables FATFS fast seek to slightly improve file read times. Thanks to @tannewt

New Boards

• SparkFun LumiDrive. Thanks to @edspark
• SparkFun Redboard Turbo. Thanks to @edspark

Breaking Changes from 3.x!

• 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.
• Improve struct compatibility with CPython by enforcing stricter types. This may break your code! Thanks to @dhalbert

Known Issues

None

Thanks

Thank you to all who used, tested, contributed, helped out, and participated on GitHub and/or Discord:
@ladyada, @dhalbert, @tannewt, @kattni, @microbuilder, @hathach, @arturo182, @nickzoic, @glennrub, @siddacious, @deshipu, @sommersoft, @jerryneedell, @brentru, @caternuson, @jepler, @urish, @sajattack, @notro, @Jahor, @Makdaam, @mrmcwethy, @jepler, @notro, @mattyt, @furlaneto, @ATMakersBill, @Klowner, @klardotsh, @C47D, @carlosperate, @platipo, @xpecex, @gcvasconcelos, @saspa, @uhrheber, @nickzoic, @sabas1080, @sajattack, @JoshuaLowe1002, @Senuros, @mchobby, @Retoc, @CedarGroveStudios, @bwshockley, @yeyeto2788, @jimblom, @rhooper, @ShawnHymel, @TG-Techie, @eighthree, @arofarn, @chalei, @vgoodwinv, @gpshead, @dsiee, @Tasm-Devil, @larsks, @CarlFK, @edspark 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.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.