Sure,
MK1 contains a conventional fuse, if a fault should occur in the vehicles wiring or alternator and excessive current flows in the circuit the fuse will rupture, this means when the fault is rectified the booster will need to be replaced same as a conventional fuse.
MK3 contains an electronic fuse, it will trip and protect the circuit if a fault occurs, when fault is rectified it resets and does not need to be replaced.
MK2 is the micro blade version of the MK3.
MK4 has same features of the MK3 plus it is user adjustable, it can be adjusted to give approximately .2V-1V in .2V steps. This is handy for those that wish to fine tune their system to match their batteries and optimise charge rates.