At 30-32 years old its time to retire the furnace... $4k is nearly the price of a new unit although you'd have to replace the DX coil and condensing unit too. Advances in energy efficiency like inverter compressors, electronically commutated motors (ECM motors), and condensing furnaces will greatly reduce your energy bills allowing you to recoup some money from the initial install.

Ducts: might be able to get away with cleaning and checking for leaks. However if they are as old as the furnace they likely have a decent amount of leaks and the insulation is not as good. Leaks and bad insulation increase your utility bills.

If your AC is also that old or 10+ years old it could be using R-22 refrigerant which was phased out long ago. Very very expensive to maintain R-22 systems. R-410A is what is being used now, but that is also being phased out within a year.