I have a 4 HP Chrysler Outboard. I dont know when the last time was that it was run. I just want to pose a hypothetical question and see if my theory is correct.

Said engine uses a 50:1 mix. But I also want to put in some Pennzoil marine fuel additive. How would that work into my mix?

Say I use a 1 gallon fuel can and put the proper fuel/oil mix to make it 50:1,then I put in the Fuel Additive per the bottle instructions,would that throw my 50:1 ratio out of whack,and change my ratio? Or how would that work. Does the additive even play a part in the mix calculation?

Not sure what type of additive your considering, I've never seen or used the one your talking about but most additives have no affect on gas/oil ratio.  It should say on the container if necessary to adjust mixture ratio.

Uh that would be a no, fuel additives should not even play a part in your "fuel/oil" calculations.