If you have four guys going at a tank...you doing something wrong. So yes a tank should be able to deal with them if the enemy is foolish enough to all focus on it...
In a standard MMO...you need someone to deal with the tank (ironically another tank in most cases or a Tank Shredding DPS who can survive or have a good support keeping them alive)...and go after the squishes he is protecting when you see the opening or viable option with the other team mates.
LoL problem is they want to make games end quicker and make things more face paced. This is why "Tanks" aren't "Tanks" in this game for the most part.
Tanks can do some damage (Not a lot, but enough to been seen as a problem if unchecked), tanks use support/aggro grabbing abilities.
The closest thing we have to a Champion acting like a true tank is Galio. A taunt (that also increases def temporally). A dash in to close gaps or stop enemy pursuers attacking ally. A minor stun (the dive in and ult). Ult is a nice concept but I would opt out of something that Boosts Team Moral (Stats) around him for a short time as an ult. Nothing really to say about his Q Poke but it helps giving a Melee at least one poke attack XD.
Shame he can get shredded because of the how game functions now and days XD