If only 4 years old, your thermocouple probably has a diode attached to it, that may be part that is actually being damaged.
. Not enough air flowing through the combustion chamber to carry the heat up through the flue causes heat build up, which kills the thermocouple. Make sure exhaust flue is large enough, not leaking, unobstructed inside, and runs in as straight and short route thru roof as possible and extends above roof far enough. Also make sure plenty of air is getting to unit to increase air flow, could be plenty of air to keep it burning but not enough to keep thermocouple cool. Larger or another opening into heater compartment, opening should be as low as possible.
Have all your replacements been same brand? Another brand may be more rugged and dependable.
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[CENTER][FONT=Comic Sans MS]Look at the nail, not the hammer. Watch the fence, not the blade.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.[/FONT][/CENTER]