I searched the app store and was only able to find "Reeder 5". As someone who hasn't purchased 4, it doesn't show up, so new users aren't confused. Old users are presumably able to figure out that the idea that the bigger number is the one they want.

As for "littering" Apple's app store, it's just rows in a database, of which there are millions. Waste sucks, but we're not talking about a meaningful amount here.