there are actually only 2 generations, there are 4 versions of the poppet valve in the mini though. Im not sure why but when invert came out with the 4th poppet someone just started saying its a gen 3 even though nothing in the marker except the valve changed.
gen 1 minis all have a 600psi guage, all came with the metal case and the poppet version 1 which had a few issues.
all currently made minis and any mini since about 06 is a gen 2 poppet 4 which is the most current, if its a new mini you are looking at, its g2p4
SOURCE:-
http://www.paintballforum.com/invert/183520-invert-mini-generation-question.html