4wd = the ability for any four wheels to drive with having all 4 wheels drive with equal traction or resistance.
Adding a locker ENHANCES 4wd by overcoming traction bias
Open diffs, most 4wd vehichles have this, this is really to protect the drivetrain on hard surfaces.
FWA Front wheel assist actually was a term years ago with a front drive unit that AUTOMATICALLY kicked in when rear wheels slipped. Ford and NH used this terminology for a short term then added an actual lever because people like to shift levers
True 4wd man made term describing axles with lockers or spools front and rear.
Limited slip = in between torque biasing between open and locked. Example is a clutch pack that creates resistance to wheel that is slipping. Doesnt work for beans if torque biasing is 100 to 0.
Tire size has nothing to do with any equation here.