The fourth Gym (third Gym in Diamond and Pearl) has gotten redone quite a lot. The puzzle has changed entirely and now you have to knock around these punching bags in order to get to the Gym Leader, Maylene. Her lineup is the same as it was, though, but her Pokemon are a bit stronger than before. You'll also have to take on some Galactic goons with Dawn (or Lucas if you're playing as a girl) afterward to reclaim her Pokedex.