Master cylinder....paint or powdercoat?

So I could go with DOT 4 and not have to worry about paint issues.

No, if you want the non-paint peeling brake fluid, use a silicone base DOT 5.
DOT 4 is usually very similar to DOT 3, being glycol based, just with a slightly higher boiling point.
There may be exceptions to this, but as a rule of thumb, it's safe to follow. Check the label for silicone base.