Rachel, here's my line of thought on tangs...
Ultimately, it's your tank so you can do what you want, so you don't have to listen to the tang police if you don't want to. Most people will tell you that Tangs are very active swimmers and require a large tank (which they do). Some people will also go one step farther and say that no tang should be in a tank less than 75g, and most should be in atleast a 125. Each tang is different, and the different species require different conditions to be happy.
For example, tangs like the powder blue, achillies, and naso are VERY active swimmers and are happiest if they have plenty of room to swim. It's my opinion that they should be kept in atleast a 6" long tank to give them ample swimming room. Again, that's just my opinion, so take it for what it's worth. Tangs like the hippo, purple, yellow and Kole aren't quite as active as some other tangs, and can usually be kept in slightly smaller tanks assuming the tank is not completely full of rock. I would have no qualms about putting any of these tangs in a 4' tank (i.e. your 75).
Here's the bottom line... it's your tank and I'm not here to tell you what to do, I'm just sharing my opinion... trying to give you more info to base your decision on. In a 4' tank with average LR, I'd say a kole, yellow, purple, or hippo would be fine. I'd also say that an achillies, powder blue or naso would be find as a juvinile fish, but the will eventually outgrow a 4' tank.
HTH.