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i rebuilt my 4l60e about 5 months ago. ever since i have had this issue. note, this trans was new to me. had no 2nd or 4th when i got it. i got screwed on it naturally so decided to rebuild.
all gears shift absolutely perfect. all gears hold solid. when im in 4th, if im cruising barely in gas, it slips out of 4th and into third. if i get on gas more it locks back in . its not commanding a shift. just comes out of 4th. it will pull a trailer up a hill in 4th and never slip. only at very low rpms it comes out. ive niticed that when the converter is unlocked, it will never do this. it only does this with converter locked. i have no idea what that has to do with anything but it does. converter feels good. if i slightly apply brake and unlock converter while its slipping out of 4th it stops and stays in 4th.i wanna say its something with the 2-4 band and servo but second is solid. never an issue. so i dont know. 3rd has 0 issues.
You are going to want to first get a line pressure gauge, and take readings.
You may have leak in forth (and have low line pressure in 4th gear).
Or you may have a worn 2-4 band/ drum.
2-4 band is new. 3-4 piston new. All clutches new. I connected hp tuners and made it so tcc cant lock and it never acts up. So i guess i lose pressure when tcc locks
Your dip-stick may be indicating the "Full Level" too low on the stick.
I would fill the oil 1/8" to 1/4" above the Oil-Pan rail.
Shown below are the typical fluid level markings on a transmission dip-stick.
These are often incorrectly market.
The dip-stick must be inspected from inside the transmission with the oil-pan removed.
To correctly identify the actual FULL mark; complete the following:
-Place a straight-edge or ruller against the transmission case oil-pan flange
-While doing so; align the straight edge with the dip-stick
-Using a "Sharpie", mark the dip-stick even with the straight-edge
-Now remove the dip-stick from the transmission and get a ruller
-Measure 1/8" (0.125") higher on the dip-stick from the "Sharpie" mark
-Scribe a new FULL mark at the 1/8" measurement on the dip-stick
-Obtain a jewelers file or other very small file for metal working
-File a small notch into one of the narrow sides of the dip-stick at the scribed line
-The notch will now indicate the correct FULL mark
-Thoroughly clean the dip-stick and continue to the ATF filling procedure
The new corrected/ actual FULL mark on the dip-stick is referred to as the "HOT FULL" mark. In reality what this means is; when the automatic transmission has fully warmed-up and reached operating temperature, this is the mark where the fluid level should be.
The ONLY way in which the automatic transmission fluid level should be checked for final confirmation... Is with the engine/ transmission fully warmed-up. As well as the transmission gear selector being in the PARK position.
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weird 4l60e 4th gear issue 
