I don't have any skin in the game, I have neither of the HUs but I've done a bunch of reversing on Pioneer's firmware and hardware/SD hacking on my unit APPRADIO 4 which has a common software and hardware base.
What's the goal? What do people actually want from the x100 series?
I took a really quick look at the differences between the 4000 / 4100 and it looks like the hardware is super similar, with a few differences:
CPU Unit
* New flash rom - this could mean the BSP has changed (and the SD passwords are different) but can't tell until inspecting the kernel or dumping the flash. The CPU hasn't changed so I'd be surprised if there were significant differences.
Interface Unit
* HD/DAB tuner change
* System uCom change
Monitor Unit
* New multi touch panel
DVD Unit
* a Mirrorlink chip is gone (or moved)
What's the same:
* CPU
* LCD + backlight
* Bascially everything (hardware version KM500B vs KM506B)
When updating things can be done independantly:
* Programs
* Bootloader
* Boot
* Recovery Partition
* Platform
* Opening Data
* System uCom firmware
* Bluetooth firmware
* GPS firmware
The code for x100 is PJ150 and x000 is PJ140.
Once updated, unless some core files are manipulated the unit will assume the new hardware's identity.
Its possible to craft an update that mixes and matches units between updates - or leave some out. It's also possible to redo a whole partition.
What this doesn't take into account is sofware and driver versions and how the harware actually talks to each other. e.g. there might many more software than hardware changes that could lead to significant interop issues. I haven't inspected the update to see (an update probbaly isn't detailed enough)
At this stage I don't see any reason to think that the tty assignments have changed on the UART interfaces though.
If I was starting on this from scratch with a x000 unit I think a good first step would be:
* back up internal SD, just in case
* make the minumum manipulations to the update to make it apply (i.e change no data just names and headers)
* once updated successfully boot into the service test mode with TESTMODE.KEY and run the hardware diagnostics - these cover basically everything, touchscreen, fans, bluetooth, monitor so should show up any issues easily.
I wouldn't force bluetooth, gps or ucom updates at this point - don't want force the HU to brick those components with an incompatible firmware.
If I had a x100 unit I'd dump the SD card and take a gander at that. (swapping unlocked sd cards would be an interesting experiment, as the firmwares wouldnt be touched just potential driver issues - if it worked it'd be funny to think you could just order the SD spare part from pioneer from a newer model to "upgrade")