Yes, you will get some strong views on tank size for certain fish such as tangs. But keep in mind why that is. Many of these people have made the mistake of keeping these fish in smaller tanks, and have either seen them die or have upgraded tanks, and seen the difference in the way the fish thrives. Experience will tell you what will and will not work long term.
Personally, I think a yellow tang will be OK in a tank with a 4 foot length. The reason for the length is to allow them room to dart back and forth. Typically, this would be a 50-55g tank minimum. However obviously, a larger tank will afford more swimming room, and thus be better long term.
Keep in mind that my view will differ from others, and some will be more passionate about their convictions than others. Take all opinions into consideration, and make your own choice.