Mysterious "5th" Gear on a 4-Speed Automatic?

Question

My 1998 Honda Civic LX is equipped with the 4-speed automatic transmission. Before I looked this up, I decided to count the number of upshifts from a stop, and I counted four, not three. This mystery "fifth gear" happens when the engine speed is around 2000 RPM in 4th and the shift brings it down to around ~1400. The kickdown for this "gear" is very sensitive – even slightly pushing on the throttle "downshifts" the car to fourth and the engine speed continues from 2000, rather than ~1400.

Why is this?

Answer 1

The transmission has a lock up torque converter.

The torque converter sits between the engine and transmission. It is kind of like a clutch on a manual transmission car. When the engine is idle, it is barely engaged, so the engine will not stall. It will engage harder and until a certain RPM. Torque converters will always have some slip in them until thy engage the lockup feature. Once the lockup is engage, there is no more slip, so the engine RPM matches the transmissions input shaft.

Answer 2

I would have assumed that this was an overdrive unit engaging and disengaging, but I found a 1998 Civic manual online, and it confirms the above answer: "Your Honda's transmission has four forward speeds, and is electronically controlled for smoother shifting. It also has a "lock-up" torque converter for better fuel economy. You may feel what seems like another shift when the converter locks." (Emphasis added) http://www.manualslib.com/manual/949320/Honda-1998-Civic-Sedan.html?page=124#manual

So on my 2002 Toyota Sienna and 2003 Toyota Corolla, when engaging and disengaging the "OD" via the button on the shifter, was that simply locking and unlocking the torque converter? Scandalous!