I'm curious, are the beans fresh?
Reason I'm asking is because I fairly recently added two fresh (scraped and split) beans to a barley wine I made. They set for about two months in the secondary and the aroma/taste was definately there. Not overpowering but way noticeable. Anyway, leaving them in for a couple months will not pose any issue. If the beans are older I'd do the four beans if they are fresh you might want to reconsider. Guess I should specify that my barley wine batch was 5 gallons. 10 gallons may take four beans.
It's a 7 gallon batch. I brewed a great Brett-Brown a few months ago; lots of flavor. This time I am doing the same beer, but intending to add vanilla and chocolate (Cocoa). It may be too much, but I'm going to give it a go.
The beans are dry, and are currently steeping in some rum to help extract flavor. Both the rum and the beans will be added to the secondary.