Your fourth kill streak slot is for killstreaks aquired from care packages.
By Completing challenges
Unfortunately no you can only have 3 kill streaks equipped at a time.
Because in C.O.D MW3 all of your kills count towards kill streak rewards. For example if after you killed the right amount of people to unlock the pred missile you can unlock the next award in your kill streak set with kills gained from using the Predator missile. Hope I helped.
To get the title 'toxic'in mw3 you must be killed by an enemy MOAB. A MOAB is a giant bomb you get in mw3 for getting a 25 kill streak. Or 24 if you are using the perk Hardline
you have to get a 25 GUN streak preferably using specialist package so you have no kill streaks and with hardline 24.
You have to use the assault strike package in order to get it. (it's a 17 kill-streak reward)
Makarav
Kill kill kill !!
you kill it with a rs-ass
It depends on what you are looking for , black ops has zombies but on mw3 it has special ops where you have to survive wave abit like zombies , in some ways its better than zombies . online on mw3 it has 3 different types of kill streak , you have support where if you dye it carrys , assualt where if you die you lose it , and you can get perk kill streaks . The missions for mw3 are a lot better than black ops . the emblem creater has gone but in my eyes .... Mw3 is better But battlefield 3 for the win.
When you kill 3 people all at one time.
use the stinger are javilin