I sounds to me like a pressure problem. For fourth gear the pump has to hold the forward clutch, the 3-4 clutch and the band. Maybe there isn't enough fluid volume to keep all this stuff pressurized due to a leak somewhere. That leak could be in the fourth apply circuit or piston as Gonzo said.

If there was no pressure loss it would be staying in third instead of going to fourth instead of going neutral. Something else is letting go during that shift either the forward clutch or the 3-4, or BOTH.

Can you tell if it's going to second when this happens? If you're going slow enough with light enough throttle you'll be able to give it enough pedal to feel it start to push the truck forward when the engine RPM comes up high enough to catch up to overrun. If it's actually falling to second and not completely neutral then it's the 3-4 clutch letting go.

EDIT:
Just saw your last post. The trans had WATER in it? No matter how long the thing sat for there should not have been water in it. I think it would be best to pressure test the ATF cooler in your radiator. That is bypassable if you have a large enough plate cooler.

 

When the truck was down before, it sat up for a full year, outside I didn t have the torque converter in it and it sat there open at the front driveshaft of the transmission all through winter. I get no coolant loss and the transmission fluid look s good.
To me, it feels like neutral, the engine revs up when it happens, if I hold my foot steady it will just coast and start to slow down,
If I let off the gas or press down on the pedal it will go back into fourth.
At this point I am totally lost about it

 

It seems like if you didn't have bad luck Randy you wouldn't have any luck at all. Sorry man.

 

I drove it home yesterday with the tech 2 and captured some data. Nothing really seemed wrong that I could tell. It says command gear 4th even when it happens. That
kinda tells me its probably not electrical. My transmission man said we would get together next week and see whats up. Good thing is, the transmission is under warranty, so i'm not that worried. May just drive it like this a little while so I can drive the truck some before we go to tearing it back apart again.

 

It seems that you and me have similar probs with similar setups. I spent $3000 trying to make 4l60e's hold. If I could turn the clocks back I would just do the '80e at first and get it over with. My advice is get that 60 fixed under warranty and then immediately sell it and put in an 80.

How is it running? Obviously good enough to smoke a trans, huh?

 

Yep, if I didnt have bad luck I just cant make myself go the 80e route. Damn thing weighs too much,lol. If I go the built trans route it will probably be a 65e.
My current transmission is fine as far as 1st,2nd,3rd go and all, it just doesnt like 4th gear
The truck is running real good so far. Gotta get the exhaust like I want it and get me a pully made for the Whipple. I went from 10psi boost to 7psi.
Gotta get my boost back
I hope both of us have better luck this go around.