How would you carb this?

Some time ago I brewed a normal gravity Abby Ale. The records were lost in a cpu crash so I can't give you the starting gravity (I believe around 1.040, but unsure), but here is the problem. They were out of the normal gravity Abby Ale yeast so I used White Labs high gravity Trappest yeast. Due to the high attenuation and alcohol tolerance it finished very dry. 1.004. I've never dealt with a beer that dry. It was fermented at a very low temperature and should be very clean. What amount of corn sugar should I prime with if it has been aging in secondary at about 74 degrees for about 3 months?

If it were mine, I'd go with my standard of about 3.7 ounces, maybe a bit less if you are concerned that the yeast will eat extra priming sugar. That is to say maybe a bit less priming sugar than you usually use. The extra attenuation is probably no more than 10% higher than other yeasts you have used, right? If it is 10% higher attenuation than a normal yeast (again, I'm just guessing here) than about 9% less priming sugar should put you spot on for a normal carbonation.

(Disclaimer: I know that math isn't perfect, but it sure is close enough for homebrew.)