Do what you like. Sometimes I use the Golden Ratio, which would have your second border finish at 3.25", but really that's up to you. The Golden Ratio is 1.618 x [size of finished inner border]; it is supposedly very pleasing to look at. But if you want to go with 4" or 6" borders, I'm sure those will also look very nice. The neat thing about wider borders is that when they are solid fabrics, I can do really fun quilting designs.
