HP-38 (aka Win231) is an extremely versatile powder. It meters well, has good case fill, shoots good in lots of pistols, has a wide load range, is fairly forgiving, etc, etc. All these contribute to making it a top choice for novice and advanced reloaders. Everybody should have a can in their stock pile. However, "perfectly clean cases" is not among its better known attributes.
Typically in my experience, it begins to shoot really good right below or at it's optimal pressure point in 9mm. The pressure inside your case is sub-optimal if you still see a black color to the interior soot. At 4.3gr it may indeed lighten up the color to tan or gray, indicating the powder is creating enough pressure to burn efficiently. However, we are loading for accuracy, not optimal burn characteristics. You might load 5-8 rounds of 4.3gr at the same OAL and bullet, and then see if accuracy and color improves. That won't hurt anything. You are not over pressure, and even if you were in +P territory the P01 can safely handle it. 4.2 to 4.3gr is where most shooters end up with HP-38 in 9mm.
If you want to see cleaner cases, then (when they return to the store shelves) you can try Alliant Sport Pistol, VihtaVuouri N320 or N330, or even Winchester WSF. If you are not a competitor, then even Hodgdon Clays is a great one.
HP-38 (aka Win231) is an extremely versatile powder with lots of good qualities for the new reloader. But it is not the end-all, ultimate, best powder in the world. It's simply easy to find, easy to get load info on, and can be used safely in numerous calibers.
All the best.