There are numerous possibilities. Burned and browned fluid may not be a good indicator if the fluid has not been changed recently. It would be good to drop the pan and check the filter, pressure control/relief valve, and look for signs of excess metal in the pan. Measure the solenoid resistances and check the electrical connections.
A new filter and refill of fluid (about 5-7 quarts) followed by a line pressure test might reveal a lot about the problem. If pressure is normal you are likely looking at a teardown. If pressure is low there may yet be hope short of a fresh transmission.
When the vehicle is first started after a cool-down, do the ranges engage quickly and firmly? Is there hesitation between Reverse and Drive? Does the transmission slip when ranges are manually selected and manually shifted?
The loss of reverse, 2nd, and 4th are symptomatic of a sun shell failure, and when it's cold it MIGHT just bite enough to provide some driving torque. Are there any unusual noises in any of the ranges?