On that page Sizemore mentions that even a 3 caster link is a real pain in the butt, and that 4 can't even be done, as far as he knows.
And I'm inclined to take what he knows, or says he knows, at face value. The guy is said to be like a rock star in the magic kingdom, and I'm sure he would've heard about 4 or more caster links if they were possible.
Sorry, man. As neat as the idea is, I don't think we could grasp how to balance it, and the current canon says that it's impossible to do more than 3.
Penty of things and pre-conceptions have been challenged lately. I'll accept the 'cantbedone' thing for now, but seriously, with casters able to bend the laws of the game itself nothing short of complete side wipeout should be undo-able, and I think can think of a way to do that last one.
For one, we know that voluntary linking is waaay less risky that normal. For two, we have a connymancer and we might not be afraid to use him.
Edit: We also have Charlie. Ooooh, baby, we have Charlie.
Offtop: Which caster do you think could influenc the stats of popped units? Before they were ven popped? Say, modifying the template of a particular unit if popped in a particular city?