What changes have been made and are the sources available? The link on the catalog page still point to the 3.0 sources and contents list 3.0 hardware.

Source is here: http://devel.lulzbot.com/TAZ/Guava/

Main changes are:

  1. Leadscrew instead of threaded rod. and changes to Z axis to accommodate that, as well as a relocated Z endstop target.
  2. Modified enclosure for the computational bits, existing enclosure will work without modification.
  3. New fully enclosed power supply. Existing power supply will work without modification.

The leadscrew and Z axis changes are really the only ones that change machine operation from the 3.1 to the 4 variant. Should improve it somewhat.
You might also want to check out the Taz mini that is under development.