I tried subdividing the bottom line to keep the edge loop for the slots from going all the way down, but it doesn't work. Am I doing something wrong, or was that removed in later versions? I'm using version 4.5.1.

Hi Shayne ttslewis123 ,
Yes, you are doing something wrong.
Most likely, you have double Vertices, so you have 2 Edges on top of each other and you Subdivided one of them, but not the other.
Exagerated:

So, Merge them (don't forget to Select at least the 4 Vertices that need Merging!) and try again.
The usual public service announcement, Press Start is not super totally beginner, it's more of a tech demo. If you want to learn topology and all that good thing, CORE mesh Modeling is the one to go first.