Thanks for the info, Carlo. But I must admit that I'm not an icon designer, due to my lags of creativity, skills and my lazyness. I did never ever create a single good looking icon.
There are already two TrueColor icon editors for OS4 available which support OS4 and PNG icons, but I never really used them yet except for some compatibility tests to check my icons.
On OS 3.9 I have the icon.datatype installed, which can load OS 3.5 icons and OS4 icons, which have been converted by my lib. PNG icons are never a problem, since any png.datatype can load them. PPaint can also handle icons, even with both images.