I personally would (and did) buy the v3.6. Even back then, exactly as now, their website stated "An initial batch of Bus Pirate v4 is now available to early adopters and testers. We think v3.x is still the best choice for most people, but v4 is the way of the future. ". That was five years ago (February 2011 to be exact). I guess the future will never come for Bus Pirate, and v3 remains the best choice.
Bus pirate is a little expensive for what it is, but I would buy another immediately if mine ever died. I really like the BASIC scripting mode, I have used this to dump entire EEPROMs in Intel Hex format, to create scrolling-message displays from cheap 8x8 LED matrix displays, and other neat tricks.
I have downloaded the source and built custom firmware for the Bus Pirate to suit my own needs. I enhanced the basic scripting mode with new commands, and I added I2C clock stretching, which the devs thought to be impossible due to a hardware errata in the I2C peripheral.... which they don't use because the I2C is bit-banged
. Serisously guys? I also significantly sped up some critical routines in the bit-bang code with inline functions and other techniques, which also reduced code size due to eliminating overhead of calling all those functions. I posted about my enhancements in their forums, and a few users seemed interested but the devs ignored my requests for help to get those enhancements into the code base, so unfortunately only I have them. Oh well.