uhhh6677, that's not always true at all. You never have a 100% chance of running into the previous Pokémon. Maybe a 50% chance, or a 95% chance, or even a 97.5% chance, but never 100%. And with the "4 steps away" thing; it's a bit more complicated than that. I could go off and write up a crude guide for people, but other websites have that already (SPPf being one).
My highest chain is 56 Flaaffy.