Transfer diagram
Trading on Generation IV / V
You can still trade on older generation DS games.
https://pkmnclassic.net/
Obtain old event Pokémon
You can obtain old event Pokémon in Gen IV / V using a DNS trick similar to the one used above for trading.
https://pokemondb.net/pokebase/383345/how-to-obtain-all-the-gen-4-5-mystery-gifts