Alpha layer is for carrying transparency information. So naturally if you put a mask in there, it automatically makes the pixels invisible. Information is still there. Inside engine, check if in your texture settings, alpha is not checked pre-multiplied. If it is set to pre-multiplied, the pixels are deleted permanently, instead of being invisible. World machine exports alpha as a separate layer.
R G B and A channels will keep their information even if your alpha makes image look super transparent, data in R G B is preserved - it is only a matter of "how you are previewing and displaying alpha channel" so you can have all 4 splatmap channels and not worry at all :)
If I do not connect anything to the alpha input of the bitmap output, everything looks fine. But I'd want a fourth channel of information. When I connect stuff into the alpha channel, the image output is transparent. There is no alpha channel, but rather a stripped RGB image.
This makes this macro a bit unusable in some cases. Or am I just thinking about this the wrong way?
In a game engine such as Unreal or Unity, you'd normally want RGB to drive where textures appear, and the Alpha channel could also drive a fourth texture, but is that possible ?