Low Attenuation mean high carbonation?

With this hobby, seems like you are always experiencing something new. Well after over a dozen batches, I ran into low attenuation and have one question please. I really appreciate your help to know what I may have done.

Information: Stout made with 1 lb of unmalted black roasted. Otherwise, just a bunch of LME to make a 5 gallon batch.

SG was 1.067, FG is 1.021. Left it 21 days in the primary hoping I would get it down further. Used Wyeast 1084 irish ale yeast. Did have a start this time, but usually I do not. It went from the 1.067 to 1.021 within 7 days. Nothing happed for the next 2 weeks. Comes out to be around 68% attenuation. I bottled them and put them in the closet at 69 degrees F.

Question: Will the priming sugar wake the yeast up and try to attenuate the beers priming sugar, PLUS the beers remaining sugars from the low attenuation? Thus causing some over carbonated beers? I used 4 oz by weight priming sugar for 4.5 gallons of bottling.

You're only a few percentage points off (71-75%). You may see a little extra carbonation from remaining sugar in the beer, but I doubt you'll end up with anything terribly out of line.