the m3 reg is for 3000 tanks, there is a m4.5 which is for 4500 - personally i don't know the difference between the new m range and g2 - but the descriptions on ansgear are exactly the same bar the m range having a lifetime fill nipple warranty.
http://www.ansgear.com/Guerrilla_Air_M3_Myth_Tank_Regulator_p/ysnguerrillamythreg3000m3.htm
http://www.ansgear.com/Guerrilla_Air_M4_5_Myth_Tank_Regulator_p/ysnguerrillamythreg4500m4.5.htm
http://www.ansgear.com/Guerrilla_Air_M3_Myth_Tank_Regulator_p/ysnguerrillamythreg3000m3.htm
http://www.ansgear.com/Guerrilla_Air_M4_5_Myth_Tank_Regulator_p/ysnguerrillamythreg4500m4.5.htm