All heaters produce condensation on the outer surfaces of parts within the unit
(sort of like a cold drink on a hot day will "sweat").

When it's cold out, the heater will produce a lot more condensation within the unit, because of the low temperature of the air circulating in the unit. So, it won't suddenly explode the first time you use it in cold wx, it just shortens its life somewhat. That's what my spa heater service rep told me. Of course if you use only use it occasionally when it's that cold out, I wouldn't "sweat" it :-) No idea why your older unit died in 4 years. Could be a number of issues.