Though internet servers have been shut down for Gen IV games, local trading is still possible. All Gen IV games require the player to obtain the Pokedex and at least two Pokemon before trading is available, but after that, you're perfectly free to do so as long as you have two DSes in proximity.
Simply enter a Pokemon Center and initiate a Union Room. If the two games are close enough to each other, both should join the same room. After that, have one game's character interact with the other, and begin a trade.
Gen IV Trading (Bulbapedia)
Union Room (Bulbapedia)
