Kind of stretching a bit maybe but this sounds a little familiar. Sort of.
4L80E's are prone to a couple of different "AFL" problems. AFL being the Actuator Feed Limit valve. It's sort of the master fluid feeder to the various shift valves. It can develop pressure leaks in various ways. The shift valves work to control gear shifts by being held in place by springs until fluid pressure is sent to them that overpowers the springs. So when fluid pressure is lacking then whichever springs start returning valves back to their starting position. Kind not very accurately oversimplifying here probably, but anyway.
In your situation I think if the AFL is leaking in 4th then the 2-3 shift valve is the one that backs off. The result is that the transmission, at least for a few seconds tries to go to first gear. it can't actually do so at 50 mph so it "overspeeds". This sounds like neutral to us. The way to catch what it is doing is to watch the computer's info of what gear it is in via a good OBD scanner that can read the datastream. If when it seems to be in neutral during the problem the computer shows it to be 1.28 ratio then that is what's happening as 1.28 is first gear.
This is all off my memory, and may not be exact in the details as the problem I was after wasn't the same as yours. And my memory isn't that good. It stuck in my head because I couldn't wrap my head around how the 2-3 shift valve could cause it to try to shift from 4 to 1.
So basically I'm saying if it were me I would be very focused on the valve body components. Sonnax has website that lists all the fixit parts they sell to fix various transmission issues. You can look there and if any symptoms they list kind of match what you are experiencing. The listings are pretty extensive. Free info. They are some of the sharpest problem solvers in the transmission industry. No actual help though, they just make parts.
Actual pro transmission help type sites want to be paid.
(Off the top of my head above except the first gear ratio. I had to look that up. I do NOT have transmission gear ratios memorized.)