Yeah, once you get to 3/4 players it helps to have an actual tank or atleast an actual healer. Going in with 2 Mages, Hunter, and dual-wielding Barbarian (or any other 4 dps combo) probably isn't the best way, no matter what the difficulty. It's why I don't run my mage with anything but the Templar in single player. I don't think that diablo forces you to use the "holy trinity" but having at least 1 of either tank or healer is helpful.
Tanks and healers doesnt really exist in the way they do in WoW so a 4 dps combo is what you will get in a 4p game, and D3 allows it, its all about your gear level when it comes to difficulty and survival comes down to your gameplay and the way you handle yourself. Ive Heard about alot of people quitting cuz the game got boring on them, and in certain aspects it does, there isnt much of a end game at all, you make your character go through the acts and level up pretty Quick and if ur not into repeating this over and over to get the gear you want it will get boring fast, I love the game, but then again im all about the challenges witch the game has plenty to offer, ive done 5 lvl 60s now and going for 10, 2 of each class, cant say im looking forward to the 100 000 000 picked up gold one but still im gonna do it. D3 is certainly not for everyone but I Think its great. As far as single player goes and what follower your using, the templar is a great pick for the wizard, but honestly the followers are not necessary at all, you'll manage fine without them, atm im running my first hardcore character, a wizard and im using the templar for healing but so far I cant say that hes made any difference but im only in act 3 nightmare so we'll see how it goes on inferno