As for the Cadillac Sixteen, 16 cylinder motors with 1000hp have been done... by Volkswagen in the Bugatti Veyron 16.4. Quad turbos in that car, too.

Since no one has really mentioned any 4-bangers, you can order a 1000hp capable Ecotec longblock from GM, supposedly. This is probably as close as you're going to get to "4-banger", "stout", and "production" being in the same sentance. Not completely on topic, but I thought I'd just throw that out there.