Cut the 4 vanilla beans into 1/2-inch pieces and grind to a coarse powder in a coffee grinder.
In a large bowl, mix the sugar, vanilla bean powder and extract until thoroughly combined (when the extract is completely incorporated the mixture will look a bit like dough). Pour the sugar mixture into an 8 by 8-inch pan and spread out evenly to a 1/2-inch thickness. Next, use a knife to score out 1/2-inch by 1/2-inch cubes by running the knife the length of the pan every 1/2-inch and the width of the pan every 1/2-inch. Bake for 1 1/2 hours. Remove the pan from oven and let cool. Once cooled, carefully flip the pan over onto a large cookie sheet to remove the sugar. Allow to cool completely, and then gently break into cubes along the scored lines.
To "round" the edges of the cubes gently pour them into a big pot with a cover and lightly shake. Check and repeat if necessary. Pour the cubes from the pot into a small holed colander or sifter and sift off any sugar particles that can be reserved as granulated vanilla bean sugar!