But if I fill it exactly to the full hot mark while hot, it starts to have a problem shifting into 4th.
--------------------------------------------------------------------------------



Does that mean it won't shift into fourth gear or shifts somehow poorly?


Yes. When it happens, it will not shift into 4th. Not manually or at any RPM.

But it is fine again. Shifts and drives excellent. Definitely no slipping, shuttering, or noises. Maybe my vent is plugged?
I'll have to locate and see. Usually happens after drive up in the mountains.

I am planning on a rebuilt trans in the spring.