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Everything seems to be going well but I have a question:

Normal shifting pattern under normal light/moderate acceleration: 1, 2, 3, 3 lock (35-40 mph), 4 w/lock (45+mph depending on accel rate).
Note it shifts into 4th immediately and locks - or stays locked. Is there such thing as 4th unlocked with the 4r100? Should it shift from 3rd into 4th unlocked then lock in 4th?

When under load downshifting out of 4th, it seems to behave similarly. There does not seem to be an unlock in 4th before it downshifts to 3rd. Seems like an unlock while remaining in 4th to get revs up would be a common sense intermediate step before going down to 3rd.

Original owner. Well maintained. 255k original trans. Trans temps are normal at around 80-100 deg above ambient. I think it has always done this. Just curious - yes first question here in 17 years of ownership.
 

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That is normal operation. It can operate in fourth gear unlocked, but not often.

You would think it is common sense to unlock in fourth and then if needed, downshift to third. I explored this at length when it was my job to make the trans shift. What I found is that in pretty much every case there is more torque to the wheels at any given speed in third gear locked than in fourth gear unlocked. So if you need more torque and unlock the converter in fourth, you will guarantee a downshift to third. It makes more sense to me (and those I worked with and for) to skip the useless unlock and just downshift to third. You'll end up there anyway, and have one less event. You will also generate less heat in the transmission.
 
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