In D3, I have a team that is 20-4 with an RPI of 13 and 48 SOS, but I am not ranked.  Is this normal??  I'm not horribly worried about rankings, but am just curious about them.

I don't know if i would say its normal but it happens, i wouldn't worry about ranking at all just your seeding in the NT.

You probably started way way down on the list because of your 4 frosh/4sophs. Early rankings are based on team strength and over the course of the season they become more oriented toward W-L. There are a lot of teams that have similar records that started out higher than you so it is tough to move up. If you do well in your CT and get past the first round of the NT, you will likely become ranked.