Should I buy the previous Metal Gear Solid games before MSG4 comes out? I don't know what MSG is about........

All I know is you play as some soldier dude with a beard and do covert ops stuff. I've seen the trailer for the 4th game and there is a dude that stabs himself to kill some other guy and I'm like what the hell is going on? And theres some robot thing jumpin around like a fruit cake for no reason. Very confusing. I'm a Halo guy.