The problem with carrots and watermelons is that the watermelon vines and leaves would block out almost all the sun the carrots would need. However, it wouldn't be completely out of the question to plant the carrots in a deep enough put, and plant the watermelons AROUND the pot altogether. At least, that's what I'd do if I was short on space. And yeah, you could probably plant the melons closer to 4 feet, but giving yourself the extra feet, in addition to re-rooting the vines as they sprawl out (pressing vine to ground, covering it with a handful of soil, placing a weight on top at least temporarily) will also increase your production.