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A gasoline engine will lose 3% of its power for every 1,000ft of elevation
So if you are at 4,000ft you're already at a 12% loss of power, and that's minimum, I would guess 15% would be more likely, as loss % increases over 3,000ft
After warm up check exhaust for Rich smell that would indicate an issue with air/fuel mix of course, and contrary to what many think Rich is bad in this situation, less power and low MPG
Find your vacuum gauge
At 4,000ft it shouldn't be lower than 16"
MAP can be tested with a Volt/ohm meter with a Hz setting, graph here: https://www.aa1car.com/library/map_sensor_ford.gif
The 2 outside wires are 5v and ground, test those first, key on
Center wire will have the Hz frequency signal the computer sees
Check key on/engine off Hz, at sea level it should be 159 Hz, at 5,000ft elevation 144 Hz, so 4,000ft should be 147 Hz
The computer gets this with key on to adjust for elevation