Those 4 inches they add to the band are 4 inches that should be in the cup size, so no wonder people think anything over DD should be huge. Still, try, by all means, but be aware that their bras may be odd compared to others. You may need to go up or down a band size, or up or down many cup sizes. A proper fit means your bra straps are not digging in. There is no adding imaginary inches here.
Samara. Age: 25.
Note that they only carry their own brand.
Leighton. Age: 27.
So what I advise is to find a store near you, try stuff on there, note down the Brand, Model, and Size, and then buy it online for up to a quarter of of the in-store price. Again, cup sizes are not static. Do not add 3 or 4 or 5 inches or whatever to your band measurement. Of course, it should not hurt either!