Ah, the old bra size debate...

The whole adding 4 inches to your band size started as nothing more than "vanity sizing": clothing manufacturers know that a lot of women have insecurities about size, so far easier to sell a 34C bra to a customer than a 30F that makes her think her perfectly normal breasts must be huge...
Of course, some companies added 4 inches to the band and shrunk the cups; some kept band sizes at +0 but made cups bigger; others increased the cup size as the band size increased, so a 32D became a '32B', a 34E became a '34B', etc. As such, nobody really knows what any particular company does, as it varies so much -- and if a store uses a rule such as +4 for all of their bras, they obviously don't know either!

I do wish all the size shaming and misconceptions around cup sizes would go away though. 'A DD cup is really big!': uh, no, for most band sizes, it really isn't.