Current Testing Situation:
I am just unable to get the original Game Boy Four player adatpers to work (with the five game paks I have now).
When using the Four Player adapter for Four Players, the title screen looks like this:

Notice the number 4 in the box on the bottom right? That's telling you how many players you have connected. It doesn't appear when you're just using two GB's with one link cable.
Plugging a second FPA into the first one, in either of the three ports doesn't allow me to do anything useful.... So I did some Google seaching and came across something that interested me.
It was a post right here on Nintendo World Report forums.
By me.
Go ahead and take a second to read the initial post in
this thread. Don't worry... I'll wait.....
....
....
Well, as you can see, for F1 Race, this particular linking method failed. Miserably. F1 Race simply would *not* recognize linked units with GBA Link cables...
...But what if FaceBall 2000 did?

Here is four GBAs, linked together in a hearty chain of love.
Booting up the units, you get:

Nothing. You'll notice there is no box saying how many players are connected, like when using the FPA.
Except, when starting the game...

"4 Players Ready" - just like it says when you start the game with the FPA.
Unfortunatly, I only have four GBA units - and thus cannot test this with a 5th one (also, I only have four GBA link cables, I'd need another one of those)... but something tells me this is the route I want to take to make this work...