Try the first link of the chain yourself. Black's row-7 three at (6,7)–(8,7) has its right end guarded by white's (10,7) — so the four is built toward the guarded side: playing (9,7) makes the four 6–9 with EXACTLY one five point, (5,7). That is not a flaw, it is the point: a four with a single completion is a move white is not allowed to answer any other way, and that is what makes a chain countable all the way to its end. Mind your ends before you fire.
Finish this step to move on.