7100 lb-rated new Silverado towing 4000 pounds

From your post, it sounds like you haven't purchased the truck yet. You would be well within the weights for the V6 model and I'm sure it would tow just fine. Out in the windy prairie and in the mountains, you will probably wish you sprung for the V8, and quite honestly, I don't think you'd see the 4% fuel economy difference in the real world with the V6.

That said, if you do get the V6, I'm very interested to hear your towing reports and the mileage you are getting.

Best of luck!