TImage32.ScaleAlpha

procedure ScaleAlpha(scale: double);

Scales the alpha byte of every pixel by the specified amount.

 uses Img32, Img32.Fmt.PNG, Img32.vector, Img32.Extra;
...
var
img: TImage32;
path: TPathD;
begin
img := TImage32.Create;
img.LOadFromFile('glowing_star.png');

img.ScaleAlpha(4);    
img.SaveToFile('extra_glowing_star.png');
img.Free;
end;

Before:

After: