Can anyone explain to me the difference between the two zoning cheats?

Apartmentbase and apartmentsublot...?

apartmentbase seems to be all that is needed to convert from residential to apartment, so what does the apartmentsublot cheat do?

I thought that we could have a total of 4 playable familes move into an apartment lot..

I have this one apartment that I'm trying to move a 4th family into, and the game won't let me.

It's a custom made lot, instead of one building, I have 4 seperate houses built on the lot.

I've been able to move a playable family into all 4 lots, just not all at once.  As soon as the 3rd family has moved in, the game only gives me the option merge the 4th family with one of the 3 that are already on the lot.  I know it's not the actual apartment, as I have moved the families in and out several times and tried all the "apartments".  But once I have 3 families there, an NPC moves into the 4th house, and I can't move the 4th family into the lot.

Anyone have any suggestions?