Bring 1/4 cup water just to a boil in a large saute pan over medium heat. Add the green beans and salt, cover and cook for 2 minutes. Uncover, raise the heat to high and cook until the water has evaporated, about 1 minute. Stir in 1 tablespoon of the butter and continue cooking until the beans are crisp-tender, 1 to 2 minutes. Remove from the heat.
Melt the remaining 3 tablespoons butter in a medium saucepan over medium heat. Add the flour and cook, stirring, until golden, about 2 minutes. Whisk in the milk, garlic powder, dry mustard, black pepper and cayenne and cook, whisking continuously, until thickened, about 2 minutes. Stir in the cheddar cheese and cream cheese and heat until melted. Pour the cheese sauce over the green beans, or serve on the side.