From the GitHub release page:

This release puts the "alpha" in our alpha releases. It features a brand-new bleio API that will definitely change over time and swaps the atmel-samd USB stack to the same one used by the nrf chips.

This is alpha release number 3 of 4.0.0. Alpha releases do not have the complete API from 3.x on the nRF builds and are only meant for testing (especially this one). (Beta release will have the full existing API but still need work.) 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, display support 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 Alpha 2

• Add bleio, the beginnings of our BLE API. Thanks @arturo182 and @dhalbert. This API will change before it is considered stable.
• Move atmel-samd to TinyUSB and refactor usb related code to be shared amongst ports. Thanks to @hathach and @tannewt. Expect to find bugs with this and please file issues for them.
• Swap the sys.path order so that frozen modules are preferred over those on the filesystem. Thanks to @dhalbert, @kattni, and @caternuson..
• Add low-level Ethernet support with network, wiznet and socket modules. Thanks to @nickzoic .
• Add .BATTERY to board for boards that allow for battery monitoring. Thanks to @sajattack.
• Add time.monotonic_ns to match the new API in Python 3.7. Thanks to @jepler
• AudioOut now preconditions the DAC at a quiescent_value to reduce audible popping when playing sound. Thanks to @tannewt, @CedarGroveStudios and @dhalbert .
• Add .channel_count and .bits_per_sample to WaveFile to make it easier to use with Mixer. Thanks to @caternuson.
• Documentation improvements thanks to @JoshuaLowe1002, @sabas1080 and @sommersoft.
• Translation updates thanks to @C47D, @sabas1080, @Senuros, @mchobby, and @Retoc.
• Update frozen modules thanks to @dhalbert, and @kattni who released many new versions of the libraries.

New Boards

• Electronic Cats’ Meow Meow. Thanks to @sabas1080
• MakerDiary NRF52840 MDK. Thanks to @klardotsh
• Fennec CP32-M4. Thanks to @siddacious
• Arduino MKR1300. Thanks to @sabas1080
• Adafruit Trellis M4 Express. Thanks to @tannewt

Breaking Changes from 3.x!

• framebuf has been removed. displayio is its replacement, and will be enhanced as 4.0 progresses to include framebuf‘s functionality.

Known Issues

• On nRF52840, using ubluepy with first allocated busio.SPI object will cause a reset. The first SPI object created is always be the high speed 32MHz SPIM3 peripheral. To avoid this, if you need SPI, create but do not use the first SPI object, and create subsequent SPI objects to use. The second and later objects will run at a maximum of 8MHz.
• Due to a bug, the SPIM3 peripheral is not allocated. Only one SPI peripheral, running at 8MHz, is available. This accidentally works around the problem above.

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 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-alpha.2. Here are all the changes since 3.1.1.

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.