Multi attack increase your primary skill by 50% MH. 4× Multi attack means 4×50%= 2. 2+1= 3×. That means your primary skill gets multiplied by 3x. Also if it's a primary skill and you use multi attack you have more animation. Flintlock would shoot 2-4 more bullets.
Extra Attack Chance gives your skill 50% chance to do an extra attack that deals 100% damage.