Can the Templar (follower) actually tank Act 3 and 4 Inferno?
Nope. The armor of your follower and damage reduction is just waaaaay too low to tank anything in inferno. Since there's no way to outfit him with armor other than a shield and what few mods you can get on jewelry, its simply not feasible. Your best bet is to give him as much +vit and magic find gear(at least you get 20% of the mf benefit) as possible and just have him soak up damage and distract enemies with his stun.