If four guys are attacking you, the study says on average it will take 2.8x4=11.2 rounds to stop all of them.
Personally, I follow the Boy Scout motto: be prepared. Be prepared for the law of averages to be on vacation out of the country when trouble knocks your door down. Always carry as many spare rounds as you can possibly stand on a daily basis. And if you have a backup gun, do the same with that. To hell with the averages.
Based on experience, my point is that IF, that is IF, you are attacked by multiple assailants, you said four.... whether they be armed with guns, knives, bats, or fists....... the actual clock time that it takes you to fire your weapon at multiple attackers = and hit them=..... YOU need to be prepared to die. Clock time will NOT allow you to shoot 4 attackers that are hell bent on destroying you.
Practice this: Have a friend stand 8 feet away from you while pretending to have a knife in his hand. Shout GO as you pull your weapon and he simultaneously takes 2 strides towards you and lunges his knife into your guts. 8 feet disappears in approximately .6 of one second. Can you pull your weapon, shoot, and disable the assailant before he punctures you? Now consider, can you stop FOUR simultaneously? Unless you are dealing with wannabe gangstas, you are a dead man.
18 rounds in your 9mm and 2 more mags might be impressive to you and your friends, but you will never be able to shoot more than 4-5 unless you are in a fire fight "at a distance".
Think about it, please. This is the real world, not a movie. You don't get to get up later and appear in the next movie.
edit: Another good practice scenerio is to draw your weapon while simultaneously falling down sideways and backwards WHILE SHOOTING UPWARDS towards your attacker. This does 2 things. 1) you are getting smaller and moving away so you are harder to hit/ as well as a moving target. 2) as you go down, your shooting trajectory is upwards which can avoid innocents who might be in the line of fire in case of a miss or an over penetration. Handy if you are confronted in a crowded area.
This also makes it fun to piss off your karate instructor when he wants to show off how he can kick a gun out of your hand for the rest of the class. (After he "died" 5 times he stormed out of the room and did not come back. After the second drop shoot, I changed and dropped to the left instead of the right// By the way, practice point shooting from the hip not sticking your arm out.... he's only a few feet away= hard to miss.).
Again, just my opinions. Hope it helps save your life.