Fourth gear places the greatest demand on the hydraulics, because both clutch packs are engaged. If there are internal leaks, pressure drops and fourth won't engage. A good shop would put a gauge set on and determine the working, modulator, and governor pressure. But bottom line, your transmission is likely on the way to a rebuild. About all you can do is throw in a bottle of snake oil and hope for the best. Look for something that includes both friction modifiers and solvents.