paypracer is mostly right, except for this part:
If the equipment said the equipped creature dealt the damage, everything but her would still die and you would gain 36 life (9 from combat to the 4/2 and 27 from the resulting triggered damage to the other 3 creatures).
..because the life you gain would depend on how much damage you assign to the blocking creature.
Because of trample you must assign lethal damage (if the 9/5 is blocked by 4/4 then = 4) to the blocker(s) then you CAN assign the rest to the player. But Kusari-Gama states that the damage done to the other creatures is equal to the damage done to the blocking creature (in this case 4)... so you would gain 9+3x4
If you want to gain more, you can assign the whole amount to the creature (9) but then you would not deal any damage to your opponent. Of course all this in the hypothetical situation in which Kusari-gama would state that your creature does the damage, thus making lifelink count.