Use \n
, like:
label4Text.set("0.0.0.0\nThis subnet mask is non-routable\nas there is no network portion")
Demo:
In [4]: s = "0.0.0.0 \nThis subnet mask is non-routable\nThere is no network portion"
In [5]: print s
0.0.0.0
This subnet mask is non-routable
There is no network portion