Hi DeathWolf - The v4 hardware design isn't finalized yet, and is still missing some key pieces like a bootloader. Once we actually have the hardware down, it will still lag behind the v3 capabilities wise until we work out a ton of new bugs.
V4 has a few new hardware features that can't be added to the v3 by software: on-board pullup voltage selection, native USB for faster binary mode stuff (when someone supports it), an EEPROM for some storage, two extra pins, and 4x the flash memory.
We'll continue to support the v3, the current firmware source supports both v3 and v4 so it's not a problem.
I wouldn't expect the v4 until next year, and then it will probably be another year before the firmware is running even partly as well as the v3. v3 will be the recommended hardware for a long time still.
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