FX-4170 is just a pre-overclocked version of FX-4100 for increased price. You can overclock the FX-4100 to 4.2GHz yourself without increasing voltages or cooling power and it will provide equal performance just like the FX-4170.
Again, buying FX-6200 is just a waste of money as it is just a pre-overclocked version of the FX-6100 and is a little bit more costlier.
What u've said is mostly true, but not entirely. Ofcource ramping up the 4100 clocks to 4.2GHz will give u the same performance as a 4170, but it will not always do it using the same amount of power draw, or within the same voltage ranges:) its a small gamble.
No 2 semi-conductor IC's are electrically identical, even if they came from the same wafer, sitting next to each other. All chips are tested against a list of parameters to determine what they end up becoming.
Logically thinking, it'd be safe to assume that the slightly better quality bulldozer chips(with respect to power draw, voltages etc), become FX 8150 or server versions, and the worst ones become FX 4100, even if they have all 4 modules working.