Yes and no, actually...
Before getting the National Pokédex, the only Pokémon from the Fourth generation that are available to capture are evolutions of pre-existing Pokémon like Yanmega or Togekiss. After that, though, many Pokémon from Sinnoh become available (for example, through Swarms, listening to Sinnoh's music on the Pokégear, the Safari Zone, and the Bug Catching contest).
Unfortunately, the Sinnoh starters need to be traded from other games regardless.