The fourth number is the alpha amount.
So for the first one
R=0.8 B=0 G=0 A=0.9
Not full red, so a little darker red, a light maroon you could say, with almost full transparency (90%)
R=0.5 B=0 G=0 A=0
Darkish red but since the alpha is 0, it's just going to be invisible.
It's worth a note, as demian said in my other topic, transparency is broken.