I had to test myself what was discussed in this thread. So I created 2 files: a PSD with Alpha (4 channels) and a PNG without (3 channels). What I found out is that the G channel is not actually better then the other channels. The Alpha is pretty much uncompressed. Furthermore, adding a 4th channel really messes with the other 3. So I will stick to 3 as much as possible. To check what I’m talking about you can download these pictures.
Again, if you have more details on what I’m probably missing please let me know.