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Would it be ok to vent a 4 inch bathroom fan through facia board?
Yes, if you can stand the appearance and the potential for leaks and the fact that you can't run a gutter now. I'd rather vent through a gable if possible. Soffit venting looks better and works well. Tommy and Norm may disagree with that, but if the soffit/ridge venting is adequate the extra moisture that does not get wafted away outside will not be a problem.
I'd rather not penetrate a roof or fascia where there is a lot of water and a high potential for water ingression- the number one killer of homes. You'll find my hatred for potential water leaks as hydrophobic as a rabid dog which is why I will not put any holes in a roof unless there is no there reasonable alternative.
Im working with a flat roof here and i definitely would not go through the flat roof. The small piece of fascia i have is tucked around the back side of the house so appearance is not an issue.
Then this should work. The only concern I'd have is if the roof drained in this area. Regardless, waterproof the vent hood well and you should be OK.
Can you vent out the wall ?
personally i hate venting through soffit. the warm moist air will rise as it exits the vent going up into the soffit space. it can be absorbed by the fascia board or underside of the fascia board which can lead to rot
If it were me, I wouldn't go through the facia for a number of reasons.
Like was said before, a well flashed and sealed roof vent shouldn't give you any trouble.