It is the texture of the skin that contibutes to the parasites ease of attaching to most tangs. This is where the prone to ich comes from. If they had a "rougher" type outer body they would not be such an easy target for the parasites.
A medium to large powder blue would not be happy in a four foot long tank for long. A small powder bue would survive for a brief time in a tank this long. They really like at least six feet of swimming space to thrive and be healthy and happy.
Yes, folks will say I have had one in a four foot tank...but keep in mine that it will grow and eventually feel cramped, stress and die. Could be a week, month are a couple years...maybe even longer but an adult blue needs more area,
Macro Algae is the best food you can feed your tangs. The powder blue also likes good steady water quality as previously posted. once they settle in they can be agressive towards their neighbors. especailly in smaller tanks.
Personally, I think a Kole tang would be a good choice for your system. They require planty greens and algae. They are like cows constantly grazing.